Add GRPCRouter

[aaronc]

ref: #5921

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For contributor use:

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer

[codecov]

Codecov Report

Merging #6325 into master will increase coverage by 0.01%.
The diff coverage is 63.63%.

@@            Coverage Diff             @@
##           master    #6325      +/-   ##
==========================================
+ Coverage   55.70%   55.71%   +0.01%     
==========================================
  Files         446      448       +2     
  Lines       26934    26978      +44     
==========================================
+ Hits        15004    15032      +28     
- Misses      10859    10869      +10     
- Partials     1071     1077       +6     

[alexanderbez]

LGTM, left a few minor comments 👍

[alexanderbez]

nit: move test types and logic to separate file.

[aaronc]

grpcrouter_helpers.go?

[alexanderbez]

ACK