perf: reduce number of queries made by /api/v2/workspaceagents/{id} (#21522)

Relates to https://github.com/coder/internal/issues/1214

The `ExtractWorkspaceAgentParam` middleware ends up making 4 database
queries to follow the chain of `WorkspaceAgent` -> `WorkspaceResource`
-> `ProvisionerJob` -> `WorkspaceBuild` -- but then dropping all that
hard work on the floor. The `api.workspaceAgent` handler that references
this middleware then has to do all of that work again, plus one more
query to get the related `User` so we can get the username. This pattern
is also mirrored in `getDatabaseTerminal` but without the middleware.

This PR:
* Adds a new query `GetWorkspaceAgentAndWorkspaceByID` to fetch all
this information at once to avoid the multiple round-trips,
* Updates the existing usage of `GetWorkspaceAgentByID` to this new
query instead,
* Updates `ExtractWorkspaceAgentParam` to also store the workspace in
the request context

Dalibo: [0.63ms](https://explain.dalibo.com/plan/40bb597f3539gc6c)

coderd/database/modelmethods.go

@@ -884,3 +884,8 @@ func WorkspaceIdentityFromWorkspace(w Workspace) WorkspaceIdentity {
 		AutostartSchedule: w.AutostartSchedule,
 	}
 }
+
+// A workspace agent belongs to the owner of the associated workspace.
+func (r GetWorkspaceAgentAndWorkspaceByIDRow) RBACObject() rbac.Object {
+	return r.WorkspaceTable.RBACObject()
+}