PoC: Duo Chat Storage with PostgreSQL (Not For Merge)

This is PoC MR. Not For Merge.

What does this MR do and why?

This MR demonstrates the change size for transitioning to PostgreSQL from Redis as Duo Chat storage. This is a preliminary work for Iteration 1: Multi-threaded conversations in th... (&15622 - closed).

The ides here is that:

• Add ai_conversation_threads and ai_conversation_messages tables to Postgresql.
• Make ai_conversation_messages (Ai::Conversation::Message AR model) compatible with Gitlab::Llm::AiMessage, so that we don't need to refactor the related components yet. Also, we can avoid blocking the work from stable counterparts (e.g. !170311 (closed)).
• We switch to PosgreSQL storage without changing the business logic. Users won't notice the difference.
• We don't migrate the data from Redis to PosgreSQL as the data will be expired in 3 days anyway.
• We should remove the Redis dependency Gitlab::Llm::ChatStorage::Redis. This is follow-up.
• We should consolidate Ai::Conversation::Message and Gitlab::Llm::AiMessage. This is follow-up.
• After these follow-ups are done, we can work on iteration 1.

If we agree upon the structure, I'll create issues in the epic.

Related to Create PoC for multi-threaded conversations (#500031 - closed)

Verification

[17] pry(main)> User.first.ai_conversation_threads.last.messages.map { |message| [message.role, message.content] }
  User Load (0.6ms)  SELECT "users".* FROM "users" ORDER BY "users"."id" ASC LIMIT 1 /*application:console,db_config_database:gitlabhq_development,db_config_name:main,console_hostname:shinya-XPS-15-9530,console_username:shinya,line:(pry):27:in `__pry__'*/
  Ai::Conversation::Thread Load (0.3ms)  SELECT "ai_conversation_threads".* FROM "ai_conversation_threads" WHERE "ai_conversation_threads"."user_id" = 1 ORDER BY "ai_conversation_threads"."id" DESC LIMIT 1 /*application:console,db_config_database:gitlabhq_development,db_config_name:main,console_hostname:shinya-XPS-15-9530,console_username:shinya,line:(pry):27:in `__pry__'*/
  Ai::Conversation::Message Load (0.3ms)  SELECT "ai_conversation_messages".* FROM "ai_conversation_messages" WHERE "ai_conversation_messages"."thread_id" = 2 /*application:console,db_config_database:gitlabhq_development,db_config_name:main,console_hostname:shinya-XPS-15-9530,console_username:shinya,line:(pry):27:in `map'*/
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