DIREGAPIC LRO implementation without annotations

[GabrielGonzalezDiaz]

Added methods to HttpJsonServiceCallableFactoryClassComposer and HttpJsonServiceStubClassComposer to generate LRO machinery without annotations. Used the established AST format.

Methods generated:

• getOperationSnapshotFactory
• setPollingRequestFactory
• createOperationCallable

TODO:
Add annotations
Find alternative way of representing generic types. Currently using a vapor reference with the same package as the class being generated.

[codecov]

Codecov Report

Merging #826 (703fc37) into diregapic-lro (a0f88b9) will decrease coverage by 1.10%.
The diff coverage is 31.92%.

@@                Coverage Diff                @@
##           diregapic-lro     #826      +/-   ##
=================================================
- Coverage          91.33%   90.22%   -1.11%     
=================================================
  Files                140      140              
  Lines              15022    15304     +282     
  Branches            1087     1088       +1     
=================================================
+ Hits               13720    13808      +88     
- Misses              1007     1200     +193     
- Partials             295      296       +1     

Impacted Files	Coverage Δ
...ic/composer/grpc/GrpcServiceStubClassComposer.java	97.87% <ø> (ø)
...omposer/rest/HttpJsonServiceStubClassComposer.java	59.32% <0.00%> (-40.68%)	⬇️
...poser/common/AbstractServiceStubClassComposer.java	98.46% <100.00%> (+<0.01%)	⬆️
...t/HttpJsonServiceCallableFactoryClassComposer.java	98.37% <100.00%> (+3.92%)	⬆️

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[vam-google]

Please make sure that the new LRO logic is generated only for the methods which return Operation. Apart from that LGTM

[vam-google]

LGTM