* Introduces amendment `featureXRPFees`
* Includes Validations, Change transactions, the "Fees" ledger object,
  and subscription messages.

* For use with CDBC and sidechain projects that do not want to require
  fees.
* Note that fee escalation logic is still in place, which may cause the
  open ledger fee to rise if the network is busy. 0 drop transactions
  will still queue, and fee escalation can be effectively disabled by
  modifying the configuration on all nodes.

* Break featureXRPCheck logic into multiple blocks.
* Change default network reserves to match mainnet.
* Add a isLegalAmountSigned check

* upstream/develop:
  Add a unit test for invalid memos (XRPLF#4287)

* upstream/develop:
  Make NodeToShardRPC a manual test (XRPLF#4379)
  Update build instructions (XRPLF#4376)

* upstream/develop:
  Update BUILD.md (XRPLF#4383)

* Pass `convertCallback` by reference.

* Clarify comments explaining the ttFEE transaction field validation.