We have the need internally to modify generated Thrift classes. This is currently
done ad-hoc with a combination of Python and sed scripting, but we can and should
be more rigorous.
This commit introduces `TypeProcessor` as an SPI, implementations of which can be
provided on the command line. Implementations will receive a JavaPoet `TypeSpec`
for every generated class, before that type is written, and must return a `TypeSpec`.
The returned type will be what is written to disk.
The returned `TypeSpec` will typically be augmented in some way - a new interface
implementation, or additional methods, or anything else one might wish.