Use protoc-gen-validate fork only for generation

[adriansmares]

Summary

References https://github.com/TheThingsIndustries/protoc-gen-validate/pull/9

Changes

• Use github.com/TheThingsIndustries/protoc-gen-validate for code generation.
• Use github.com/envoyproxy/protoc-gen-validate for protobuf registration and definitions.
• Update dependencies.