Register x/mint gRPC gateway routes

[mdaleem]

Description

ref: #5921
follow-up: #7019

---

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 #7214 into master will decrease coverage by 0.02%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #7214      +/-   ##
==========================================
- Coverage   54.68%   54.66%   -0.03%     
==========================================
  Files         569      571       +2     
  Lines       39248    39297      +49     
==========================================
+ Hits        21463    21481      +18     
- Misses      16011    16039      +28     
- Partials     1774     1777       +3     

[amaury1093]

lgtm