[Feature Request] Go API for fetching predefined constraints

[sudorandom]

Feature description:
protovalidate-go has an API for easily fetching field/message/oneof constraints in the resolver package. I propose that we add a similar API for getting predefined constraints in a similar way.

I see that there is code in the cache to fetch them but it seems like there’s not an exposed way to do something similar.

Problem it solves or use case:
It's useful for me to get protovalidate constraints in order to annotate generated OpenAPI specifications from protobuf. I suppose it may be useful for others as well.

Proposed implementation or solution:
ResolveFieldConstraints, ResolveMessageConstraints and ResolveOneofConstraints could be augmented to resolve the predefined constraints from extensions containing validate.E_Predefined.

Contribution:
protobuf extensions are pretty crazy to me, but I can help if there's no movement on this after a while.

Examples or references:
https://pkg.go.dev/github.com/bufbuild/[email protected]/resolver

Additional context:

[jchadwick-buf]

Thinking about it a bit, at the moment, there's no particular reason for the StandardConstraintResolver interface to actually have a method for resolving the predefined constraint fields. However, maybe in spite of that, we could just provide a ResolvePredefinedConstraints method on DefaultResolver anyhow, just for convenience. If there's no objections, that's probably what I'll wind up doing.