add tests & refactor tx crate - alt (backport #3868)

[mergify]

Describe your changes

an alternative to #3835 (without the refactor of tx sections access 9842883, which breaks compatibility with hermes/gaia in a way I don't yet understand)

related to #1497

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 #3868 done by [Mergify](https://mergify.com).

[codecov]

Codecov Report

Attention: Patch coverage is 81.89386% with 174 lines in your changes missing coverage. Please review.

Project coverage is 73.21%. Comparing base (2bd9a60) to head (59db279).

Files with missing lines	Patch %	Lines
crates/tx/src/section.rs	69.00%	137 Missing ⚠️
crates/tx/src/sign.rs	80.99%	23 Missing ⚠️
crates/tx/src/types.rs	98.39%	6 Missing ⚠️
crates/node/src/shell/finalize_block.rs	37.50%	5 Missing ⚠️
crates/node/src/shell/mod.rs	33.33%	2 Missing ⚠️
crates/node/src/shell/prepare_proposal.rs	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           0.44.0    #3871      +/-   ##
==========================================
+ Coverage   73.04%   73.21%   +0.17%     
==========================================
  Files         340      341       +1     
  Lines      104782   105150     +368     
==========================================
+ Hits        76536    76987     +451     
+ Misses      28246    28163      -83     

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