chore: add hardcoded LRO implementation for rest transport

[GabrielGonzalezDiaz]

Adding hardcoded machinery to composer/rest

Changes

HttpJsonServiceStubClassComposer

Added 2 new methods to generate .setSnapshotFactory() and .setPollingRequestFactory() using the AST model already established.

Problem:
This version is currently hard coded as it does not use the new annotations for LRO.

HttpJsonServiceCallableFactoryClassComposer

Modified 1 method to generate createOperationCallable() using the AST model already established. The previous version generated the header of the method with an empty body and returned null. I took the already generated MethodDefinition to add the variables needed in the body for the return statement.

Problem:
Generic types in the generics for a new object are not currently supported, so to get around that I created a VaporReference with the same pakkage as the class being generated such that the import will be ignored. This is a temporary solution.

[vam-google]

Please include the baseline and integration test files changes (you added changes in the generator, so now it generates new stuff, and that new stuff now must be reflected in the expected baseline and integration test files).

[vam-google]

For every line of code, which has now compute hardcoded, add a corresponding TODO (that will be easy to search for , when we will be replacing the hardcoded parts).