Re-randomize after each send

[yinyiqian1]

• Add receiver-side ciphertext re-randomization for ConfidentialMPTSend

1. The destination encrypted amount is re-randomized by adding Enc_dest(0; e)
2. The issuer-encrypted mirror amount is also re-randomized by adding Enc_issuer(0; e)

• Added rerandomizeCiphertext in ConfidentialTransfer.cpp to encrypt zero and homomorphically adds it to an existing ciphertext.
• Added test testSendRerandomizesRecipientInboxAgainstMergeCancellation to test the attack senario described in TOB-RIPCTXR-5.

High Level Overview of Change

Context of Change

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

❌ Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 82.3%. Comparing base (9854b3e) to head (b10ba12).
⚠️ Report is 12 commits behind head on ripple/confidential-transfer.

Files with missing lines	Patch %	Lines
src/libxrpl/protocol/ConfidentialTransfer.cpp	80.0%	1 Missing ⚠️

Additional details and impacted files

@@                      Coverage Diff                       @@
##           ripple/confidential-transfer   #7535     +/-   ##
==============================================================
+ Coverage                          81.6%   82.3%   +0.7%     
==============================================================
  Files                              1026    1026             
  Lines                             78124   78143     +19     
  Branches                           9309    8941    -368     
==============================================================
+ Hits                              63729   64287    +558     
+ Misses                            14386   13847    -539     
  Partials                              9       9             

Files with missing lines	Coverage Δ
include/xrpl/protocol/ConfidentialTransfer.h	100.0% <ø> (+100.0%)	⬆️
...bxrpl/tx/transactors/token/ConfidentialMPTSend.cpp	93.2% <100.0%> (+93.2%)	⬆️
src/libxrpl/protocol/ConfidentialTransfer.cpp	94.0% <80.0%> (+48.2%)	⬆️

... and 18 files with indirect coverage changes

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[shawnxie999]

exclude line cov

[yinyiqian1]

Sorry, I just merged the code. Would you mind adding it in the next PR?