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/rbac/roles.go

@@ -299,6 +299,8 @@ func ReloadBuiltinRoles(opts *RoleOptions) {
 				ResourceOrganizationMember.Type: {policy.ActionRead},
 				// Users can create provisioner daemons scoped to themselves.
 				ResourceProvisionerDaemon.Type: {policy.ActionRead, policy.ActionCreate, policy.ActionRead, policy.ActionUpdate},
+				// Users can create, read, update, and delete their own agentic chat messages.
+				ResourceChat.Type: {policy.ActionCreate, policy.ActionRead, policy.ActionUpdate, policy.ActionDelete},
 			})...,
 		),
 	}.withCachedRegoValue()