feat: add database tables and API routes for agentic chat feature (#17570)

Backend portion of experimental `AgenticChat` feature:
- Adds database tables for chats and chat messages
- Adds functionality to stream messages from LLM providers using
`kylecarbs/aisdk-go`
- Adds API routes with relevant functionality (list, create, update
chats, insert chat message)
- Adds experiment `codersdk.AgenticChat`

---------

Co-authored-by: Kyle Carberry <kyle@carberry.com>

coderd/database/queries/chat.sql

@@ -0,0 +1,36 @@
+-- name: InsertChat :one
+INSERT INTO chats (owner_id, created_at, updated_at, title)
+VALUES ($1, $2, $3, $4)
+RETURNING *;
+
+-- name: UpdateChatByID :exec
+UPDATE chats
+SET title = $2, updated_at = $3
+WHERE id = $1;
+
+-- name: GetChatsByOwnerID :many
+SELECT * FROM chats
+WHERE owner_id = $1
+ORDER BY created_at DESC;
+
+-- name: GetChatByID :one
+SELECT * FROM chats
+WHERE id = $1;
+
+-- name: InsertChatMessages :many
+INSERT INTO chat_messages (chat_id, created_at, model, provider, content)
+SELECT
+    @chat_id :: uuid AS chat_id,
+    @created_at :: timestamptz AS created_at,
+    @model :: VARCHAR(127) AS model,
+    @provider :: VARCHAR(127) AS provider,
+    jsonb_array_elements(@content :: jsonb) AS content
+RETURNING chat_messages.*;
+
+-- name: GetChatMessagesByChatID :many
+SELECT * FROM chat_messages
+WHERE chat_id = $1
+ORDER BY created_at ASC;
+
+-- name: DeleteChat :exec
+DELETE FROM chats WHERE id = $1;