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/migrations/000319_chat.down.sql

@@ -0,0 +1,3 @@
+DROP TABLE IF EXISTS chat_messages;
+
+DROP TABLE IF EXISTS chats;

coderd/database/migrations/000319_chat.up.sql

@@ -0,0 +1,17 @@
+CREATE TABLE IF NOT EXISTS chats (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    owner_id UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    title TEXT NOT NULL
+);
+
+CREATE TABLE IF NOT EXISTS chat_messages (
+    -- BIGSERIAL is auto-incrementing so we know the exact order of messages.
+    id BIGSERIAL PRIMARY KEY,
+    chat_id UUID NOT NULL REFERENCES chats(id) ON DELETE CASCADE,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    model TEXT NOT NULL,
+    provider TEXT NOT NULL,
+    content JSONB NOT NULL
+);

coderd/database/migrations/testdata/fixtures/000319_chat.up.sql

@@ -0,0 +1,6 @@
+INSERT INTO chats (id, owner_id, created_at, updated_at, title) VALUES
+('00000000-0000-0000-0000-000000000001', '0ed9befc-4911-4ccf-a8e2-559bf72daa94', '2023-10-01 12:00:00+00', '2023-10-01 12:00:00+00', 'Test Chat 1');
+
+INSERT INTO chat_messages (id, chat_id, created_at, model, provider, content) VALUES
+(1, '00000000-0000-0000-0000-000000000001', '2023-10-01 12:00:00+00', 'annie-oakley', 'cowboy-coder', '{"role":"user","content":"Hello"}'),
+(2, '00000000-0000-0000-0000-000000000001', '2023-10-01 12:01:00+00', 'annie-oakley', 'cowboy-coder', '{"role":"assistant","content":"Howdy pardner! What can I do ya for?"}');