C# routing header support

[chrisdunelm]

No description provided.

[codecov-io]

Codecov Report

Merging #1629 into master will increase coverage by 0.52%.
The diff coverage is 50%.

@@             Coverage Diff              @@
##             master    #1629      +/-   ##
============================================
+ Coverage     85.51%   86.04%   +0.52%     
+ Complexity     4268     4214      -54     
============================================
  Files           405      403       -2     
  Lines         16787    16635     -152     
  Branches       1757     1735      -22     
============================================
- Hits          14355    14313      -42     
+ Misses         1849     1742     -107     
+ Partials        583      580       -3

Impacted Files	Coverage Δ	Complexity Δ
.../google/api/codegen/viewmodel/ApiCallableView.java	71.42% <0%> (-11.91%)	6 <2> (+2)
...pi/codegen/transformer/ApiCallableTransformer.java	95.49% <100%> (ø)	35 <0> (ø)	⬇️
...i/codegen/transformer/SchemaTypeNameConverter.java	36.36% <0%> (-16.97%)	1% <0%> (-2%)
...pi/codegen/transformer/ModelTypeNameConverter.java	75% <0%> (-5%)	6% <0%> (-2%)
...i/codegen/transformer/DiscoGapicMethodContext.java	86.95% <0%> (-4.35%)	9% <0%> (-2%)
.../transformer/java/JavaSchemaTypeNameConverter.java	48.03% <0%> (-3.39%)	21% <0%> (-3%)
...google/api/codegen/transformer/ModelTypeTable.java	85.1% <0%> (-3.03%)	30% <0%> (-9%)
.../com/google/api/codegen/metacode/InitCodeNode.java	79.89% <0%> (-2.67%)	73% <0%> (+2%)
...google/api/codegen/config/PageStreamingConfig.java	37.25% <0%> (-1.8%)	10% <0%> (ø)
...oogle/api/codegen/transformer/SchemaTypeTable.java	64.44% <0%> (-1.69%)	18% <0%> (-5%)
... and 61 more

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[chrisdunelm]

I've built pubsub with this change, and confirmed it builds.

[chrisdunelm]

This requires an as-yet-unreleased version of gax; so will require syncing with gax and dependencies.yaml.

[jskeet]

I've mostly looked at the generated code changes, which look nice and simple.