APIv2(DeliverMax): add alias for Amount in Payment transactions (#4733)

Using the "Amount" field in Payment transactions can cause incorrect
interpretation. There continue to be problems from the use of this
field. "Amount" is rarely the correct field to use; instead,
"delivered_amount" (or "DeliveredAmount") should be used.

Rename the "Amount" field to "DeliverMax", a less misleading name. With
api_version: 2, remove the "Amount" field from Payment transactions.

- Input: "DeliverMax" in `tx_json` is an alias for "Amount"
  - sign
  - submit (in sign-and-submit mode)
  - submit_multisigned
  - sign_for
- Output: Add "DeliverMax" where transactions are provided by the API
  - ledger
  - tx
  - tx_history
  - account_tx
  - transaction_entry
  - subscribe (transactions stream)
- Output: Remove "Amount" from API version 2

Fix #3484

Fix #3902

Builds/CMake/RippledCore.cmake

@@ -217,6 +217,7 @@ install (
 install (
   FILES
     src/ripple/json/JsonPropertyStream.h
+    src/ripple/json/MultivarJson.h
     src/ripple/json/Object.h
     src/ripple/json/Output.h
     src/ripple/json/Writer.h
@@ -467,6 +468,7 @@ target_sources (rippled PRIVATE
   src/ripple/app/misc/detail/impl/WorkSSL.cpp
   src/ripple/app/misc/impl/AccountTxPaging.cpp
   src/ripple/app/misc/impl/AmendmentTable.cpp
+  src/ripple/app/misc/impl/DeliverMax.cpp
   src/ripple/app/misc/impl/LoadFeeTrack.cpp
   src/ripple/app/misc/impl/Manifest.cpp
   src/ripple/app/misc/impl/Transaction.cpp
@@ -918,6 +920,7 @@ if (tests)
     src/test/json/Output_test.cpp
     src/test/json/Writer_test.cpp
     src/test/json/json_value_test.cpp
+    src/test/json/MultivarJson_test.cpp
     #[===============================[
        test sources:
          subdir: jtx