Reduce event's masp attributes

[grarco]

Describe your changes

Closes #3824.

Reduces the number of a tx event's masp attributes down to a single one.

Checklist before merging

• If this PR has some consensus breaking changes, I added the corresponding breaking:: labels
  • This will require 2 reviewers to approve the changes
• If this PR requires changes to the docs or specs, a corresponding PR is opened in the namada-docs repo
  • Relevant PR if applies:
• If this PR affects services such as namada-indexer or namada-masp-indexer, a corresponding PR is opened in that repo
  • Relevant PR if applies: Single masp event attribute namada-masp-indexer#22

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.82%. Comparing base (28ebf30) to head (297ee26).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3826      +/-   ##
==========================================
- Coverage   72.82%   72.82%   -0.01%     
==========================================
  Files         338      338              
  Lines      104230   104220      -10     
==========================================
- Hits        75909    75901       -8     
+ Misses      28321    28319       -2     

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