fix!: add Amino support for x/subspaces messages

[RiccardoM]

Description

This PR adds Amino encoding supporrt for x/subspaces messages

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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
• targeted the correct branch (see PR Targeting)
• provided a link to the relevant issue or specification
• followed the guidelines for building modules
• included the necessary unit and integration tests
• 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

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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 ! in the type prefix if API or client breaking change
• 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)

[codecov]

Codecov Report

Merging #849 (53504b0) into master (376a0be) will increase coverage by 0.14%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #849      +/-   ##
==========================================
+ Coverage   81.32%   81.47%   +0.14%     
==========================================
  Files          91       91              
  Lines        7229     7232       +3     
==========================================
+ Hits         5879     5892      +13     
+ Misses       1075     1065      -10     
  Partials      275      275              

Impacted Files	Coverage Δ
x/subspaces/types/codec.go	48.14% <100.00%> (+48.14%)	⬆️

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