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 | null;
    after: xdr.LedgerEntry | null;
  }

[github-actions]

Size Change: +9.12 kB (+0.08%)

Total Size: 12 MB

Filename	Size	Change
dist/stellar-sdk.js	6.68 MB	+5.96 kB (+0.09%)
dist/stellar-sdk.min.js	5.28 MB	+3.16 kB (+0.06%)

_{compressed-size-action}

[Shaptic]

Don't forget a CHANGELOG.md entry!

[Shaptic]

One last comment. Great test coverage btw! 👏

[Shaptic]

Doing it this way means stateChanges is always present, and might be set as stateChanges: undefined. We want to omit the property altogether. This is why we need the spread operator. Observe the differences:

$ node         
> {
  value: [].length > 0 ? "hello" : undefined
}
{ value: undefined }
> // vs.
> {
  ...([].length > 0 ?? { value: "hello" })
}
{}

[psheth9]

Yes I think we are on same page for stateChanges, I thought you were proposing the same for before and after property which seemed wrong but gotchu now :)

[Shaptic]

I was but I changed my mind - we should stick to the RPC API 👍