Add a single CometBFT dummy validator

[sug0]

Describe your changes

Closes #4096

Only writes a dummy validator to CometBFT's genesis if the number of validators present is lower than 2.

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 8 lines in your changes missing coverage. Please review.

Project coverage is 74.57%. Comparing base (c61bbe0) to head (1021625).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/node/src/tendermint_node.rs	0.00%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4116      +/-   ##
==========================================
- Coverage   74.58%   74.57%   -0.01%     
==========================================
  Files         341      341              
  Lines      107491   107495       +4     
==========================================
  Hits        80169    80169              
- Misses      27322    27326       +4     

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[grarco]

I believe this closes #4096 unless we want to remove that command anyway