Enforce gas spending key usage with disposable gas payers

[sug0]

Describe your changes

Restore commits accidentally purged from #3959 (more specifically, acafd29)

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 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 73.88%. Comparing base (0e36167) to head (a871c33).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/shielded_token/src/masp/shielded_wallet.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3979      +/-   ##
==========================================
- Coverage   73.95%   73.88%   -0.07%     
==========================================
  Files         341      341              
  Lines      106442   106444       +2     
==========================================
- Hits        78716    78647      -69     
- Misses      27726    27797      +71     

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