Add transaction and ledger entry formats to server_definitions

[mvadari]

Summary

The server_definitions RPC returns all the fields, transaction types, ledger entry types, etc. that the server knows about. This PR suggests adding the transaction formats and ledger formats to this RPC as well.

Motivation

This will provide a canonical source of truth for the transactions and ledger entry type fields.

Solution

Amend the server_definitions RPC to include this info.

Paths Not Taken

You can parse source code as-is (for example, this script in xrpl.js does so). There may be an OpenAPI-related path as well.

[dangell7]

We could also add the tx flags

[donovanhide]

Ledger Entry Flags too :-)

[thejohnfreeman]

I prefer the source code solution as well, with the build system generating the corresponding header and implementation files for xrpld. I just don't have the time personally to devote to building it out. It can be a great project for a new contributor or intern though.