feature: XDR serialization of Auth Entries for multi-auth workflow

[BlaineHeffron]

We currently have toJSON and fromJSON methods in AssembledTransaction, as well as txFromJSON in ContractClient, and typed fromJSON methods in the TS Bindings generation in soroban-cli.

This was a first-pass at supporting multi-auth workflows, and is not needed by most use-cases. For multi-auth workflows, we want to support a full-XDR serialization approach, rather than unexpectedly using JSON in this one Soroban use-case.

We need to design a multi-auth workflow that potentially just does toXDR on the specific authEntries to be signed, which could then be sent to other users/machines, then signed and re-serialized, then sent back to the original transaction author, on whose machine it can then be reassembled and submitted.