fix: support reroute_to_grpc_interface with grpc_service_config

[noahdietz]

This fixes a bug when supplying a grpc_service_config for retry settings and also using the reroute_to_grpc_interface GAPIC config option. Only two APIs are known to be using this option and this change was tested against them.

The approach is slightly hacky, passing the GAPIC interface name through that declares the method being rerouted, and overriding the rerouted service name with the GAPIC interface name when looking up the retry_params assignment. The retry configs name these rerouted RPCs using the same name as the GAPIC interface name.

[codecov]

Codecov Report

Merging #3272 into master will decrease coverage by 0.00%.
The diff coverage is 83.33%.

@@             Coverage Diff              @@
##             master    #3272      +/-   ##
============================================
- Coverage     87.12%   87.12%   -0.01%     
- Complexity     6080     6081       +1     
============================================
  Files           494      494              
  Lines         24060    24066       +6     
  Branches       2613     2615       +2     
============================================
+ Hits          20962    20967       +5     
  Misses         2236     2236              
- Partials        862      863       +1     

Impacted Files	Coverage Δ	Complexity Δ
...m/google/api/codegen/config/GapicMethodConfig.java	90.34% <80.00%> (-0.37%)	25.00 <0.00> (+1.00)	⬇️
...oogle/api/codegen/config/GapicInterfaceConfig.java	81.02% <100.00%> (+0.13%)	49.00 <0.00> (ø)

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[vam-google]

LGTM