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/policy/policy.go

@@ -104,6 +104,14 @@ var RBACPermissions = map[string]PermissionDefinition{
 			ActionRead:   actDef("read and use a workspace proxy"),
 		},
 	},
+	"chat": {
+		Actions: map[Action]ActionDefinition{
+			ActionCreate: actDef("create a chat"),
+			ActionRead:   actDef("read a chat"),
+			ActionDelete: actDef("delete a chat"),
+			ActionUpdate: actDef("update a chat"),
+		},
+	},
 	"license": {
 		Actions: map[Action]ActionDefinition{
 			ActionCreate: actDef("create a license"),