Protovalidate interceptor cleanup, Go version bump

[ash2k]

Changes

Please see individual commits.

Verification

Unit tests. No significant changes.

[ash2k]

Follow up questions:

• I have a custom codec that I use with gRPC. It handles both proto and non-proto (a single type) messages. I'd like to use the protovalidate interceptors with it. Would you accept a feature to specify a list of Go types (as reflect.Type) to ignore, instead of erroring on them.
• Would you accept a PR with client validating interceptors?

[johanbrandhorst]

* I have a custom codec that I use with gRPC. It handles both proto and non-proto (a single type) messages. I'd like to use the protovalidate interceptors with it. Would you accept a feature to specify a list of Go types (as `reflect.Type`) to ignore, instead of erroring on them.

No, I'd rather users create their own interceptors for this sort of thing.

* Would you accept a PR with client validating interceptors?

For outgoing requests? For responses from servers? Not sure where that would go.

[johanbrandhorst]

Thanks for your contribution!

[ash2k]

Thank you!