Enhancements for binapigen

[ondrej-fabry]

This PR contains enhancements for the binapi-generator and binapigen package. The aim was to finish the implementation of generator plugins and allow adding custom plugins by users.

This change contains the following changes:

• [x] adds support for running external plugins in generator
  • external plugin is a binary (*.so) compiled from Go main package using go build -buildmode=plugin ...
  • this Go package must contain at least one exported function:
  • GenerateFile(g *binapigen.Generator, f *binapigen.File) *binapigen.GenFile (runs once for each file)
  • GenerateAll(g *binapigen.Generator) []*binapigen.GenFile (runs only once for all)
  • the binapi-generator accepts path to an external plugin and loads its symbol (via plugin package)
• [x] adds universal input reference URI to the generator, resolving the VPP input parameters depending on the format
  • supporte formats are:
  • path to directory to VPP API files (e.g. /usr/share/vpp/api)
  • path to directory containing VPP repository (runs make json-api-files automatically)
  • (more formats will be added in the future..)
• [x] adds new helper functions and constructors to generated code for converting address types like Address, MacAddress, Prefix, .. into types from net package (IP, IPNet...)
• [x] minor cleanups/refactorings of code in binapigen package

This PR includes collective changes from Nathan's PRs: #67 #68

There is a discussion created for design and overall info for this feature: https://github.com/FDio/govpp/discussions/94

[sknat]

lgtm ! Thanks 😄