Add support for unknown TransactionType

[rsteimen]

Add an UnknownTransaction object that can take the place of a transaction that xrpl4j doesn't yet support. For example, loading transactions from Xahau should not blow-up (but currently they would).

[sappenin]

Hi @rsteimen - thanks for filing this issue, and for your PR. Overall, I like the problem you're solving for, and need to review your PR to consider things more deeply (e.g., I think if we want to support this kind of change, we may want to align how we handle unknown LedgerObjects with the pattern/APIs you proposed).

Overall, I don't have a timeline for you here -- but wanted to "ack" your submission. Thanks for proposing it!