plugin system

[guilhem]

New Features:

• Added new plugin system using https://github.com/hashicorp/go-plugin/
• Add a dumb plugin example

Code Consistency and Refactoring:

• Move types in pkg/store in their own package
  • prevent import cycle in plugin system.

Dependency Updates:

• Updated several dependencies google.golang.org/grpc, and google.golang.org/protobuf [1] [2] [3].

Linting and Formatting

• move go vet and gofmt/goimports checks in golangci-lint.
  • prevent checking generated files
• update github.com/google/addlicense
  • ignore some files like yaml files.

[danielfoehrKn]

Thank you very much and sorry for the delay. I'll release soon.