FEATURE: add custom fields to chat (channel/message/thread) (#29504)

This allows various extensions to store extra information the 3 most popular
chat entities

Useful for certain plugins and migrations

plugins/chat/db/migrate/20241031050638_add_custom_fields_to_chat.rb

@@ -0,0 +1,30 @@
+# frozen_string_literal: true
+
+class AddCustomFieldsToChat < ActiveRecord::Migration[7.1]
+  def change
+    create_table :chat_thread_custom_fields do |t|
+      t.bigint :thread_id, null: false
+      t.string :name, limit: 256, null: false
+      t.string :value, limit: 1_000_000
+      t.timestamps null: false
+    end
+
+    create_table :chat_message_custom_fields do |t|
+      t.bigint :message_id, null: false
+      t.string :name, limit: 256, null: false
+      t.string :value, limit: 1_000_000
+      t.timestamps null: false
+    end
+
+    create_table :chat_channel_custom_fields do |t|
+      t.bigint :channel_id, null: false
+      t.string :name, limit: 256, null: false
+      t.string :value, limit: 1_000_000
+      t.timestamps null: false
+    end
+
+    add_index :chat_thread_custom_fields, %i[thread_id name], unique: true
+    add_index :chat_message_custom_fields, %i[message_id name], unique: true
+    add_index :chat_channel_custom_fields, %i[channel_id name], unique: true
+  end
+end