Add a basic command handling framework inspired by Bukkit/BungeeCord.

This doesn't yet support tab complete, that will come later. Additionally,
a /server command (using your configuration) and /velocity (shows basic
copyright information about the proxy) have been added.

api/src/main/java/com/velocitypowered/api/command/CommandExecutor.java

@@ -0,0 +1,29 @@
+package com.velocitypowered.api.command;
+
+import com.google.common.collect.ImmutableList;
+
+import javax.annotation.Nonnull;
+import java.util.List;
+
+/**
+ * Represents a command that can be executed by a {@link CommandInvoker}, such as a {@link com.velocitypowered.api.proxy.Player}
+ * or the console.
+ */
+public interface CommandExecutor {
+    /**
+     * Executes the command for the specified {@link CommandInvoker}.
+     * @param invoker the invoker of this command
+     * @param args the arguments for this command
+     */
+    void execute(@Nonnull CommandInvoker invoker, @Nonnull String[] args);
+
+    /**
+     * Provides tab complete suggestions for a command for a specified {@link CommandInvoker}.
+     * @param invoker the invoker to run the command for
+     * @param currentArgs the current, partial arguments for this command
+     * @return tab complete suggestions
+     */
+    default List<String> suggest(@Nonnull CommandInvoker invoker, @Nonnull String[] currentArgs) {
+        return ImmutableList.of();
+    }
+}

api/src/main/java/com/velocitypowered/api/command/CommandInvoker.java

@@ -0,0 +1,23 @@
+package com.velocitypowered.api.command;
+
+import net.kyori.text.Component;
+
+import javax.annotation.Nonnull;
+
+/**
+ * Represents something that can be used to run a {@link CommandExecutor}.
+ */
+public interface CommandInvoker {
+    /**
+     * Sends the specified {@code component} to the invoker.
+     * @param component the text component to send
+     */
+    void sendMessage(@Nonnull Component component);
+
+    /**
+     * Determines whether or not the invoker has a particular permission.
+     * @param permission the permission to check for
+     * @return whether or not the invoker has permission to run this command
+     */
+    boolean hasPermission(@Nonnull String permission);
+}

api/src/main/java/com/velocitypowered/api/proxy/InboundConnection.java

@@ -0,0 +1,26 @@
+package com.velocitypowered.api.proxy;
+
+import java.net.InetSocketAddress;
+
+/**
+ * Represents a connection to the proxy. There is no guarantee that the connection has been fully initialized.
+ */
+public interface InboundConnection {
+    /**
+     * Returns the player's IP address.
+     * @return the player's IP
+     */
+    InetSocketAddress getRemoteAddress();
+
+    /**
+     * Determine whether or not the player remains online.
+     * @return whether or not the player active
+     */
+    boolean isActive();
+
+    /**
+     * Returns the current protocol version this connection uses.
+     * @return the protocol version the connection uses
+     */
+    int getProtocolVersion();
+}

api/src/main/java/com/velocitypowered/api/proxy/Player.java

@@ -1,18 +1,18 @@
 package com.velocitypowered.api.proxy;
 
+import com.velocitypowered.api.command.CommandInvoker;
 import com.velocitypowered.api.server.ServerInfo;
 import com.velocitypowered.api.util.MessagePosition;
 import net.kyori.text.Component;
 
 import javax.annotation.Nonnull;
-import java.net.InetSocketAddress;
 import java.util.Optional;
 import java.util.UUID;
 
 /**
  * Represents a player who is connected to the proxy.
  */
-public interface Player {
+public interface Player extends CommandInvoker, InboundConnection {
     /**
      * Returns the player's current username.
      * @return the username
@@ -31,18 +31,6 @@ public interface Player {
      */
     Optional<ServerInfo> getCurrentServer();
 
-    /**
-     * Returns the player's IP address.
-     * @return the player's IP
-     */
-    InetSocketAddress getRemoteAddress();
-
-    /**
-     * Determine whether or not the player remains online.
-     * @return whether or not the player active
-     */
-    boolean isActive();
-
     /**
      * Sends a chat message to the player's client.
      * @param component the chat message to send

api/src/main/java/com/velocitypowered/api/proxy/ProxyServer.java

@@ -1,5 +1,6 @@
 package com.velocitypowered.api.proxy;
 
+import com.velocitypowered.api.command.CommandInvoker;
 import com.velocitypowered.api.server.ServerInfo;
 
 import javax.annotation.Nonnull;
@@ -62,4 +63,12 @@ public interface ProxyServer {
      * @param server the server to unregister
      */
     void unregisterServer(@Nonnull ServerInfo server);
+
+    /**
+     * Returns an instance of {@link CommandInvoker} that can be used to determine if the command is being invoked by
+     * the console or a console-like executor. Plugins that execute commands are strongly urged to implement their own
+     * {@link CommandInvoker} instead of using the console invoker.
+     * @return the console command invoker
+     */
+    CommandInvoker getConsoleCommandInvoker();
 }