Add basic x/bank gRPC query service

[aaronc]

ref: #5921

This adds a basic gRPC Query service for x/bank implemented on the keeper.

It does not handle pagination (#6191), grpc-gateway REST mappings, or update the query CLI cmd's - those are to be added in future PRs,

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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 #6343 into master will increase coverage by 0.02%.
The diff coverage is 78.37%.

@@            Coverage Diff             @@
##           master    #6343      +/-   ##
==========================================
+ Coverage   55.70%   55.72%   +0.02%     
==========================================
  Files         449      450       +1     
  Lines       27006    27036      +30     
==========================================
+ Hits        15043    15067      +24     
- Misses      10883    10886       +3     
- Partials     1080     1083       +3     

[fedekunze]

LGTM

[alexanderbez]

ACK