[Feature Request] Validation rules on methods about protovalidate

[derekperkins]

I'm not sure if this fits under this issue or if it's something separate, but I'm interested in having separate validation for multiple methods using the same proto message. We generally follow Google's AIP recommendations, and for Create/Update calls, they specifically use the same message in both request objects. That generally works great, but often validation for create has different constraints than for an update. To solve this, it seems like it can be approached in a few ways:

1. Per this issue, have separate validation rules on the method
2. Add validation rules to the separate request messages
3. Make the calling method available as a CEL argument

2 seems like the most maintainable, though it would really benefit from shared / reusable rules. Thoughts?