feat: serde rename batch index into masp tx index (backport #3860)

[mergify]

Describe your changes

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:

---

This is an automatic backport of pull request #3860 done by [Mergify](https://mergify.com).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.24%. Comparing base (328540c) to head (7aaf26f).
Report is 15 commits behind head on 0.44.0.

Additional details and impacted files

@@            Coverage Diff             @@
##           0.44.0    #3905      +/-   ##
==========================================
- Coverage   73.25%   73.24%   -0.02%     
==========================================
  Files         341      341              
  Lines      105241   105241              
==========================================
- Hits        77098    77086      -12     
- Misses      28143    28155      +12     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.