Migrate x/capability to Protobuf

[fedekunze]

ref: #5444

Description

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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

---

For admin use:

• Added appropriate labels to PR (ex. WIP, R4R, docs, etc)
• Reviewers assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

Merging #5926 into ibc-alpha will increase coverage by 0.00%.
The diff coverage is 80.64%.

@@            Coverage Diff             @@
##           ibc-alpha    #5926   +/-   ##
==========================================
  Coverage      59.17%   59.18%           
==========================================
  Files            397      397           
  Lines          22982    22981    -1     
==========================================
+ Hits           13600    13601    +1     
+ Misses          8456     8454    -2     
  Partials         926      926           

[alexanderbez]

LGTM 👍

[cwgoes]

utACK