Introduce raw String-based command API

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

@@ -0,0 +1,59 @@
+package com.velocitypowered.api.command;
+
+import com.google.common.collect.ImmutableList;
+import java.util.List;
+import org.checkerframework.checker.nullness.qual.NonNull;
+
+/**
+ * A specialized sub-interface of {@code Command} which indicates that the proxy should pass a
+ * raw command to the command. This is useful for bolting on external command frameworks to
+ * Velocity.
+ */
+public interface RawCommand extends Command {
+  /**
+   * Executes the command for the specified {@link CommandSource}.
+   *
+   * @param source the source of this command
+   * @param commandLine the full command line after the command name
+   */
+  void execute(CommandSource source, String commandLine);
+
+  default void execute(CommandSource source, String @NonNull [] args) {
+    execute(source, String.join(" ", args));
+  }
+
+  /**
+   * Provides tab complete suggestions for a command for a specified {@link CommandSource}.
+   *
+   * @param source the source to run the command for
+   * @param currentLine the current, partial command line for this command
+   * @return tab complete suggestions
+   */
+  default List<String> suggest(CommandSource source, String currentLine) {
+    return ImmutableList.of();
+  }
+
+  @Override
+  default List<String> suggest(CommandSource source, String @NonNull [] currentArgs) {
+    return suggest(source, String.join(" ", currentArgs));
+  }
+
+  default boolean hasPermission(CommandSource source, String @NonNull [] args) {
+    return hasPermission(source, String.join(" ", args));
+  }
+
+  /**
+   * Tests to check if the {@code source} has permission to use this command with the provided
+   * {@code args}.
+   *
+   * <p>If this method returns false, the handling will be forwarded onto
+   * the players current server.</p>
+   *
+   * @param source the source of the command
+   * @param commandLine the arguments for this command
+   * @return whether the source has permission
+   */
+  default boolean hasPermission(CommandSource source, String commandLine) {
+    return true;
+  }
+}