fix: gov endpoints fail to decode old leverage proposals after upgrade

[toteki]

Description

This will revert some file deletions from #1486 and register a concrete type - hopefully that is sufficient to fix broken endpoints.

closes: #1627

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Author Checklist

All items are required. Please add a note to the item if the item is not applicable and
please add links to any relevant follow up issues.

I have...

• included the correct type prefix in the PR title
• added ! to the type prefix if API or client breaking change
• added appropriate labels to the PR
• targeted the correct branch (see PR Targeting)
• provided a link to the relevant issue or specification
• added a changelog entry to CHANGELOG.md
• included comments for documenting Go code
• updated the relevant documentation or specification
• reviewed "Files changed" and left comments if necessary
• confirmed all CI checks have passed

Reviewers Checklist

All items are required. Please add a note if the item is not applicable and please add
your handle next to the items reviewed if you only reviewed selected items.

I have...

• confirmed the correct type prefix in the PR title
• confirmed all author checklist items have been addressed
• reviewed state machine logic
• reviewed API design and naming
• reviewed documentation is accurate
• reviewed tests and test coverage
• manually tested (if applicable)

[toteki]

Not tested yet but please have a look

[codecov-commenter]

Codecov Report

Merging #1628 (9b0c24f) into main (f468a4c) will decrease coverage by 0.19%.
The diff coverage is 9.67%.

@@            Coverage Diff             @@
##             main    #1628      +/-   ##
==========================================
- Coverage   54.05%   53.86%   -0.20%     
==========================================
  Files          71       72       +1     
  Lines        7150     7181      +31     
==========================================
+ Hits         3865     3868       +3     
- Misses       2988     3016      +28     
  Partials      297      297              

Impacted Files	Coverage Δ
x/leverage/types/gov.go	8.00% <8.00%> (ø)
x/leverage/types/codec.go	42.85% <16.66%> (-5.42%)	⬇️

[gsk967]

LGTM

[robert-zaremba]

We migrate gov proposals in #1640 , hence this is not needed.