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/queries/workspaceagents.sql

@@ -393,3 +393,28 @@ AND wb.build_number = (
     WHERE wb2.workspace_id = w.id
 )
 AND workspace_agents.deleted = FALSE;
+
+-- name: GetWorkspaceAgentAndWorkspaceByID :one
+SELECT
+	sqlc.embed(workspace_agents),
+	sqlc.embed(workspaces),
+	users.username as owner_username
+FROM
+	workspace_agents
+JOIN
+	workspace_resources ON workspace_agents.resource_id = workspace_resources.id
+JOIN
+	provisioner_jobs ON workspace_resources.job_id = provisioner_jobs.id
+JOIN
+	workspace_builds ON provisioner_jobs.id = workspace_builds.job_id
+JOIN
+	workspaces ON workspace_builds.workspace_id = workspaces.id
+JOIN
+	users ON workspaces.owner_id = users.id
+WHERE
+	workspace_agents.id = @id
+	AND workspace_agents.deleted = FALSE
+	AND provisioner_jobs.type = 'workspace_build'::provisioner_job_type
+	AND workspaces.deleted = FALSE
+	AND users.deleted = FALSE
+LIMIT 1;