Refactor Gax::GRPC::Stub to a class

[blowmage]

Implement calling the RPC by calling a method on the Stub object instead of creating an ApiCall object. This allows us to fix the code that calls APIs in Gax instead of in the generator.

Generator support added in googleapis/gapic-generator-ruby#152.

[codecov[bot]]

Codecov Report

Merging #206 into master will increase coverage by 0.05%. The diff coverage is 28.57%.

@@            Coverage Diff             @@
##           master     #206      +/-   ##
==========================================
+ Coverage   28.33%   28.38%   +0.05%     
==========================================
  Files          44       44              
  Lines        2566     2572       +6     
==========================================
+ Hits          727      730       +3     
- Misses       1839     1842       +3

Impacted Files	Coverage Δ
lib/google/gax/paged_enumerable.rb	28.2% <25%> (-0.37%)	:arrow_down:
lib/google/gax/grpc/stub.rb	34.48% <29.41%> (+5.31%)	:arrow_up:

---

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[codecov[bot]]

Codecov Report

Merging #206 into master will increase coverage by 0.05%. The diff coverage is 27.27%.

@@            Coverage Diff             @@
##           master     #206      +/-   ##
==========================================
+ Coverage   28.33%   28.38%   +0.05%     
==========================================
  Files          44       44              
  Lines        2566     2572       +6     
==========================================
+ Hits          727      730       +3     
- Misses       1839     1842       +3

Impacted Files	Coverage Δ
lib/google/gax/paged_enumerable.rb	28.2% <25%> (-0.37%)	:arrow_down:
lib/google/gax/grpc/stub.rb	34.48% <27.77%> (+5.31%)	:arrow_up:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 40e9181...cd1aa9b. Read the comment docs.