## Summary
Fixes incomplete MPT holder indexing in ETL.
Previously, Clio only indexed MPT holders when an `ltMPTOKEN` ledger
node was created by an `MPTokenAuthorize` transaction. However, with
future amendments, XRPL can also create `ltMPTOKEN` nodes as part of
other successful transactions, such as `Payment`, when a holder receives
an MPT. Those holders were silently skipped and therefore missing from
the `mpt_holders` RPC results.
This change makes MPT holder indexing metadata-driven instead of
transaction-type-driven. ETL now scans successful transaction metadata
for newly created `ltMPTOKEN` nodes and writes every holder it finds.
## What Changed
- MPT holder extraction now works for any successful transaction that
creates an `ltMPTOKEN`.
- Multiple MPT holders created in a single transaction are all indexed.
- The `mpt_holders` table is kept consistent with ledger metadata
instead of relying on assumptions about which transaction types can
create holder nodes.
- Added regression coverage for MPT holders created by a `Payment`
transaction.
## Testing
- `build/clio_tests '--gtest_filter=MPTExtTests.*'`
- `clang-tidy -p build src/etl/MPTHelpers.cpp src/etl/MPTHelpers.hpp
src/etl/impl/ext/MPT.cpp tests/unit/etl/ext/MPTTests.cpp`
