add json validation for plugins recipe

This CL adds json validation ofr plugins recipe.

Bug: https://github.com/flutter/flutter/issues/56143
Change-Id: I34b8a26803caf6414556a4cdd8dd9c9a7eaffdc8
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/6281
Reviewed-by: Godofredo Contreras <godofredoc@google.com>
Commit-Queue: Keyong Han <keyonghan@google.com>