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/modelmethods.go

@@ -568,3 +568,8 @@ func (m WorkspaceAgentVolumeResourceMonitor) Debounce(
 
 	return m.DebouncedUntil, false
 }
+
+func (c Chat) RBACObject() rbac.Object {
+	return rbac.ResourceChat.WithID(c.ID).
+		WithOwner(c.OwnerID.String())
+}