feat: Allow workspace resources to attach multiple agents (#942)

This enables a "kubernetes_pod" to attach multiple agents that
could be for multiple services. Each agent is required to have
a unique name, so SSH syntax is:

`coder ssh <workspace>.<agent>`

A resource can have zero agents too, they aren't required.

coderd/database/queries/workspaceresources.sql

@@ -23,8 +23,7 @@ INSERT INTO
 		transition,
 		address,
 		type,
-		name,
-		agent_id
+		name
 	)
 VALUES
-	($1, $2, $3, $4, $5, $6, $7, $8) RETURNING *;
+	($1, $2, $3, $4, $5, $6, $7) RETURNING *;