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

[Adphi]

The plugin would add proto options:

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

Code generation:

• 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 b5784a8