Decode XDR structures wherever possible.

[Shaptic]

Is your feature request related to a problem? Please describe.
It's annoying to decode base64-encoded XDR that comes from the raw JSON responses because:

(a) you don't necessarily know what the type is without referencing the documentation
(b) you need to actually write the decoding code and error-check it

Describe the solution you'd like
This should be part of Servers request methods: they should wrap and decode the raw responses wherever possible:

• simulateTransaction
• getTransaction
• sendTransaction
• getLedgerEntries
• getLatestLedger
• getEvents

Additional context
The work for this has been started in #127 and #129.

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