Add API for custom chat completions like on Paper (#1232)

* Add API for custom chat completions like on Paper, renamed enum constant to match its function, NMS and wiki-vg

* add null and version checks

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

@@ -309,4 +309,33 @@ public interface Player extends
    * @return the player's client brand
    */
   @Nullable String getClientBrand();
+
+
+  /**
+   * Add custom chat completion suggestions shown to the player while typing a message.
+   *
+   * @param completions the completions to send
+   */
+  void addCustomChatCompletions(@NotNull Collection<String> completions);
+
+  /**
+   * Remove custom chat completion suggestions shown to the player while typing a message.
+   *
+   * <p>Online player names can't be removed with this method, it will only affect
+   * custom completions added by {@link #addCustomChatCompletions(Collection)}
+   * or {@link #setCustomChatCompletions(Collection)}.
+   *
+   * @param completions the completions to remove
+   */
+  void removeCustomChatCompletions(@NotNull Collection<String> completions);
+
+  /**
+   * Set the list of chat completion suggestions shown to the player while typing a message.
+   *
+   * <p>If completions were set previously, this method will remove them all
+   * and replace them with the provided completions.
+   *
+   * @param completions the completions to set
+   */
+  void setCustomChatCompletions(@NotNull Collection<String> completions);
 }