Add support for unknown transactions

[nkramer44]

Adds a new field to Transaction called unknownFields, which is annotated with @JsonAnySetter. This tells Jackson to populate unknownFields with any JSON fields that are unmapped in the immutable.

This allow for two things:

1. Out of the box support for any new transaction fields that the current version/an old version of xrpl4j doesn't support
2. The introduction of UnknownTransaction, which only has common fields + unknownFields, which allows any version of xrpl4j that contains UnknownTransaction to deserialize an unsupported transaction type from JSON

[codecov]

Codecov Report

Attention: Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 92.01%. Comparing base (613339d) to head (51589f4).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
.../xrpl4j/model/transactions/UnknownTransaction.java	66.66%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #569      +/-   ##
============================================
- Coverage     92.02%   92.01%   -0.01%     
- Complexity     1843     1848       +5     
============================================
  Files           382      383       +1     
  Lines          5115     5125      +10     
  Branches        435      435              
============================================
+ Hits           4707     4716       +9     
- Misses          272      273       +1     
  Partials        136      136              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[sappenin]

LGTM -- mostly cleanup and docs suggestions, but otherwise good to go. Thanks for adding support here!