Add protoc-gen-protodb: protoc plugin to generate boilerplate client grpc services code

[Adphi]

The plugin would add proto options:

• Messages:
  • [x] ignore messages
  • [x] automatically register messages types
  • [x] mark field as message key
  • [ ] mark field as index
  • [ ] mark field as pointer / relation
• Services:
  • [ ] Generate CRUD handlers

Code generation:

• [x] embedded client typed code (automatically un/marshal from/to google.protobuf.Any)
• [ ] typed field filters
• [ ] CRUD handlers
• Messages hooks:
  • [ ] before read
  • [ ] after read
  • [ ] before write
  • [ ] after write
  • [ ] before delete
  • [ ] after delete

[Adphi]

patially done in b5784a8298fdce9a9b45e520396cd676b1dc6cfa