*: Add the support for structured annotations

[AlexisMontagne]

Thrift annotations on thrift files, yes you heard it right.

This is the successor of #12 that is a generalization. Instead of modifying the thrift AST everytime we want to add a side concept to the type / service definition this PR is here to generalize this concept.

This allows to add annotation (thrift struct) over:

• types
• services
• fields
• methods.

That can be consumed by either:

• the compiler directly for codegen (i have some side project of a plugin system).
• the runtime (most likely at framework level) using reflection ( you can see the utilisation in go & rb)