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

[nkramer44]

@sappenin we should discuss this change

[sappenin]

Well, if we wanted to keep the @Derived could make a special carve-out in the Deserializer to add Account into EXTRA_TRANSACTION_FIELDS.

However, that's ugly, and makes it so nobody can create a UnlModify with an account that's not account-zero. That's a non-goal ATM, but I think I prefer making this @Default just in case someone every wants to do that (e.g., for testing, or whatever).

[codecov]

Codecov Report

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

Project coverage is 92.04%. Comparing base (18f9dab) to head (3041912).

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     92.04%   92.04%   -0.01%     
- Complexity     1842     1847       +5     
============================================
  Files           382      383       +1     
  Lines          5106     5116      +10     
  Branches        433      433              
============================================
+ Hits           4700     4709       +9     
- Misses          271      272       +1     
  Partials        135      135              

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[sappenin]

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

[sappenin]

Well, if we wanted to keep the @Derived could make a special carve-out in the Deserializer to add Account into EXTRA_TRANSACTION_FIELDS.

However, that's ugly, and makes it so nobody can create a UnlModify with an account that's not account-zero. That's a non-goal ATM, but I think I prefer making this @Default just in case someone every wants to do that (e.g., for testing, or whatever).