binapigen cli improvements

[sknat]

This patch also proposes an evolution to the binapigen CLI in order to make it more user-friendly & discoverable and allow build pipelines based on go:generate

For now two usecases are in the picture:

• Building from a directory containing .json files e.g. 'binapigen --api /usr/share/vpp/api --output ./myapp/bindings --filter interface,ip'
• Building from a cloned local VPP e.g. 'binapigen --vpp ~/vpp --output ./myapp/binding'