avoid running cometbft as a validator when tendermint_node is set to non-validator

[tzemanovic]

Describe your changes

closes #4139

The PR adds a new field in config last_tendermint_mode (non-breaking as it's optional) which will be persisted on node start-up. If the mode changes from validator to non-validator, the node will backup and replace the validator key and state in the CometBFT dir.

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 2.94118% with 66 lines in your changes missing coverage. Please review.

Project coverage is 74.55%. Comparing base (9eb747c) to head (c802a29).
Report is 53 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/node/src/lib.rs	0.00%	43 Missing ⚠️
crates/apps_lib/src/tendermint_node.rs	0.00%	23 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4245      +/-   ##
==========================================
- Coverage   74.59%   74.55%   -0.04%     
==========================================
  Files         342      342              
  Lines      108771   108832      +61     
==========================================
+ Hits        81135    81138       +3     
- Misses      27636    27694      +58     

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[grarco]

lgtm, thanks!