feat(agent): add ParentId to agent manifest (#17888)

Closes https://github.com/coder/internal/issues/648

This change introduces a new `ParentId` field to the agent's manifest.
This will allow an agent to know if it is a child or not, as well as
knowing who the owner is.

This is part of the Dev Container Agents work

coderd/workspaceagents_test.go

@@ -2575,7 +2575,7 @@ func requireGetManifest(ctx context.Context, t testing.TB, aAPI agentproto.DRPCA
 }
 
 func postStartup(ctx context.Context, t testing.TB, client agent.Client, startup *agentproto.Startup) error {
-	aAPI, _, err := client.ConnectRPC24(ctx)
+	aAPI, _, err := client.ConnectRPC25(ctx)
 	require.NoError(t, err)
 	defer func() {
 		cErr := aAPI.DRPCConn().Close()