[JVPP-13] Refactor Java generator as vppapigen plugin

[vvalderrv]

The vppapigen was recently rewritten in python:

https://gerrit.fd.io/r/#/c/8781/

 

It has modular architecture which might allow adding Java api generation as a plugin.

Advantages:

• no need for intermediate JSON representation
• share AST with other language bindings

 

Issues:

 * vppapigen plugins are expected to transform list of objects to a string, which is then written to single file. For Java API generator, this should not work, as we produce single file per transfer object for example... But we could at least share Semantic Model