Minimum validator commission rate param

[brentstone]

Describe your changes

Adds a PoS param to enforce a minimum allowed validator commission rate.

Also includes logic in init_chain to set a validator's commission rate to the minimum if the one provided in the toml file is below.

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 68.88889% with 14 lines in your changes missing coverage. Please review.

Project coverage is 73.07%. Comparing base (760462b) to head (637a0a0).

Files with missing lines	Patch %	Lines
crates/sdk/src/tx.rs	41.66%	14 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3831      +/-   ##
==========================================
+ Coverage   73.01%   73.07%   +0.05%     
==========================================
  Files         340      340              
  Lines      104750   104791      +41     
==========================================
+ Hits        76488    76571      +83     
+ Misses      28262    28220      -42     

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