add tests & refactor tx crate

[tzemanovic]

Describe your changes

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:

[codecov]

Codecov Report

Attention: Patch coverage is 78.84131% with 168 lines in your changes missing coverage. Please review.

Project coverage is 72.94%. Comparing base (be38d4c) to head (5e283f6).

Files with missing lines	Patch %	Lines
crates/tx/src/section.rs	68.09%	141 Missing ⚠️
crates/tx/src/sign.rs	80.99%	23 Missing ⚠️
crates/tx/src/types.rs	98.26%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3835      +/-   ##
==========================================
+ Coverage   72.82%   72.94%   +0.11%     
==========================================
  Files         338      339       +1     
  Lines      104220   104454     +234     
==========================================
+ Hits        75897    76191     +294     
+ Misses      28323    28263      -60     

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