Add StateChanges in SimulateTransaction API response

[psheth9]

Fixes #936

This Diff has following changes:

• Add stateDiff in SimulateTransaction API response where stateDiff interface is below
• Add parsing of XDR types to build SimulateTxnResponse from Raw encoded XDR
• Add unit tests

  interface LedgerEntryDiff{
    type: number;
    key: xdr.LedgerKey;
    before?: xdr.LedgerEntry;
    after?: xdr.LedgerEntry;
  }

[github-actions]

Size Change: +9.92 kB (+0.08%)

Total Size: 12 MB

Filename	Size	Change
dist/stellar-sdk.js	6.69 MB	+6.52 kB (+0.1%)
dist/stellar-sdk.min.js	5.28 MB	+3.4 kB (+0.06%)

_{compressed-size-action}

[Shaptic]

Don't forget a CHANGELOG.md entry!

[Shaptic]

if you want before and after to be omitted when they aren't present you'll need to use the spread operator (check in this file for examples, I think it's something like

key: xdr.LedgerKey.fromXDR(entryChange.key, 'base64'),
...(entryChange.before && { before: xdr.LedgerEntry.fromXDR(entryChange.before, 'base64') },

and you want this for stateChanges too. Alternatively you can use delete (much easier to read lol).

[psheth9]

I think now there is no need to omit as these are required fields. so spread operator wont be needed here

[Shaptic]

What I mean is that maybe we want to omit them at the SDK level since the API choice of making them null is odd (imo).

[Shaptic]

This should have a bit more detail, e.g. at least the schema change and mention rpc.Server.simulateTransaction

[Shaptic]

I think we can get away with not exporting this one?

[Shaptic]

wait shouldn't this fail to decode to a LedgerKey?