Disallow mutation when COA not empty

[yinyiqian1]

• Disallow mutation when confidential outstanding amount is larger than 0

High Level Overview of Change

Context of Change

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactor (non-breaking change that only restructures code)
• Performance (increase or change in throughput and/or latency)
• Tests (you added tests for code that already exists, or your new feature included in this PR)
• Documentation update
• Chore (no impact to binary, e.g. .gitignore, formatting, dropping support for older tooling)
• Release

API Impact

• Public API: New feature (new methods and/or new fields)
• Public API: Breaking change (in general, breaking changes should only impact the next api_version)
• libxrpl change (any change that may affect libxrpl or dependents of libxrpl)
• Peer protocol change (must be backward compatible or bump the peer protocol version)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (ripple/confidential-transfer@b94c95b). Learn more about missing BASE report.

Additional details and impacted files

@@                      Coverage Diff                       @@
##             ripple/confidential-transfer   #6091   +/-   ##
==============================================================
  Coverage                                ?   78.3%           
==============================================================
  Files                                   ?     827           
  Lines                                   ?   69594           
  Branches                                ?    8368           
==============================================================
  Hits                                    ?   54492           
  Misses                                  ?   15102           
  Partials                                ?       0           

Files with missing lines	Coverage Δ
src/xrpld/app/tx/detail/MPTokenIssuanceSet.cpp	98.9% <100.0%> (ø)

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[shawnxie999]

My apologies since the ticket wasn't too clear. The ticket was meant to also capture the changes the our discussion around mutability - having tfMPTCanConfidentialTransfer and lsmfMPTCannotMutateCanConfidentialTransfer, where the issuer can mutate the flag by default

Maybe tfMPTPrivacy and lsmfMPTCannotMutatePrivacy are better names