x/Slashing: gRPC query service

[sahith-narahari]

Description

Ref: #5921

---

Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• 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
• Review Codecov Report in the comment section below once CI passes

[codecov]

Codecov Report

Merging #6597 into master will decrease coverage by 7.34%.
The diff coverage is 63.63%.

@@            Coverage Diff             @@
##           master    #6597      +/-   ##
==========================================
- Coverage   57.66%   50.32%   -7.35%     
==========================================
  Files         493      242     -251     
  Lines       29621    13125   -16496     
==========================================
- Hits        17082     6605   -10477     
+ Misses      11314     6010    -5304     
+ Partials     1225      510     -715     

[alexanderbez]

utACK

[amaury1093]

lgtm, +1 for the tests