Partial veneer for PHP

[michaelbausor]

cc @dwsupplee @jdpedrie

This PR splits the generated GAPIC client into two files:

• <API>Gapic: the implementation
• <API>Client: an empty class which extends <API>Gapic and can be used to hold any partial veneer methods

An update to the Vision API is shown in this PR: googleapis/google-cloud-php#600

[codecov-io]

Codecov Report

Merging #1428 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master    #1428      +/-   ##
============================================
+ Coverage     87.16%   87.17%   +0.01%     
- Complexity     4035     4039       +4     
============================================
  Files           386      387       +1     
  Lines         16216    16231      +15     
  Branches       1770     1770              
============================================
+ Hits          14135    14150      +15     
  Misses         1619     1619              
  Partials        462      462

Impacted Files	Coverage Δ	Complexity Δ
...m/google/api/codegen/transformer/SurfaceNamer.java	77.56% <ø> (ø)	182 <0> (ø)	⬇️
...ansformer/csharp/CSharpGapicClientTransformer.java	98.77% <100%> (ø)	38 <0> (ø)	⬇️
...codegen/viewmodel/DynamicLangXApiSubclassView.java	100% <100%> (ø)	3 <3> (?)
...codegen/transformer/csharp/CSharpSurfaceNamer.java	92.51% <100%> (ø)	72 <1> (ø)	⬇️
...en/transformer/php/PhpGapicSurfaceTransformer.java	98.57% <100%> (+0.09%)	15 <1> (ø)	⬇️
...e/api/codegen/transformer/php/PhpSurfaceNamer.java	87.75% <100%> (+0.25%)	28 <1> (+1)	⬆️

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[shinfan]

LGTM