Add MPT support to DEX

[gregtatcam]

High Level Overview of Change

Add MPT support to DEX

Context of Change

Implements XLS-82d

Adds MPT support for the following transactions:

• Payment
• OfferCreate
• CheckCash
• All AMM transactions
• Pathfinding

Retires FlowCross feature.

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)

Test Plan

Extended MPToken unit-test to test for basic MPT interactions with DEX.

Significant number of unit-tests must be added to test all aspects of MPT interactions with DEX. Essentially all current unit-tests designed to test IOU interactions with DEX must be replicated for MPT. This is work in progress.

[codecov]

Codecov Report

Attention: Patch coverage is 85.52328% with 314 lines in your changes missing coverage. Please review.

Project coverage is 78.1%. Comparing base (3a55a64) to head (5134103).
Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/xrpld/app/paths/detail/MPTEndpointStep.cpp	74.4%	86 Missing ⚠️
src/libxrpl/protocol/STParsedJSON.cpp	52.0%	24 Missing ⚠️
src/xrpld/app/paths/PathRequest.cpp	78.7%	19 Missing ⚠️
src/xrpld/rpc/BookChanges.h	0.0%	18 Missing ⚠️
src/xrpld/app/tx/detail/CashCheck.cpp	84.4%	17 Missing ⚠️
src/xrpld/app/tx/detail/AMMCreate.cpp	85.5%	11 Missing ⚠️
src/xrpld/app/tx/detail/AMMWithdraw.cpp	88.2%	10 Missing ⚠️
src/xrpld/app/tx/detail/InvariantCheck.cpp	82.0%	9 Missing ⚠️
src/xrpld/app/tx/detail/Transactor.cpp	55.0%	9 Missing ⚠️
include/xrpl/protocol/AmountConversions.h	77.1%	8 Missing ⚠️
... and 32 more

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #5285     +/-   ##
=========================================
- Coverage     78.2%   78.1%   -0.1%     
=========================================
  Files          790     793      +3     
  Lines        67639   68233    +594     
  Branches      8161    8256     +95     
=========================================
+ Hits         52865   53260    +395     
- Misses       14774   14973    +199     

Files with missing lines	Coverage Δ
include/xrpl/protocol/AMMCore.h	100.0% <ø> (ø)
include/xrpl/protocol/Asset.h	97.3% <100.0%> (+2.5%)	⬆️
include/xrpl/protocol/Feature.h	100.0% <ø> (ø)
include/xrpl/protocol/Indexes.h	100.0% <ø> (ø)
include/xrpl/protocol/MPTAmount.h	87.9% <100.0%> (-12.1%)	⬇️
include/xrpl/protocol/MPTIssue.h	100.0% <100.0%> (ø)
include/xrpl/protocol/STObject.h	92.9% <ø> (ø)
include/xrpl/protocol/TER.h	100.0% <ø> (ø)
include/xrpl/protocol/detail/ledger_entries.macro	100.0% <ø> (ø)
include/xrpl/protocol/detail/transactions.macro	100.0% <ø> (ø)
... and 92 more

... and 7 files with indirect coverage changes

[shawnxie999]

haven't reviewed the changes but one invariant would be good to have is to ensure that the amount changed for OutstandingBalance in MPTokenIssuance should equal the net change of all the MPTokens modified in the transaction