refactor mpt_issuance_id into SLE::getJson

[shawnxie999]

High Level Overview of Change

Missing mpt_issuance_id in ledger_entry response if the object type is MPTokenIssuance. Although not necessary (since the user probably already knows the mpt id in their request), we still should make things consistent by injecting the id into the response if it's a MPTokenIssuance object.

Context of Change

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)

[gregtatcam]

Somehow this doesn't feel right. We did specialized parsing above and now we do it again.

[shawnxie999]

The specialized parsing above is only to find which object index given the request params. Here we are injecting the synthetic field after we've retrieved the object from DB. So I do believe this is the right place.

[shawnxie999]

But, I think we can refactor this. I think i can move this code to STLedgerEntry, let me refactor it

[gregtatcam]

Sure. But we have already parsed all ledger entries and we are of doing it again. What if more ledger entries require this special processing then we pretty much have to repeat specialized type parsing again. Maybe we could have a global json variable, which is set during the ledger entry parsing and then the values from this variable are copied into sleJson?