Update js-stellar-sdk

[jeesunikim]

• Update js-stellar-sdk:
• Check XDR-JSON package
• Create a P23 feature branch

Following xdr methods have been updated. For example, invoke contract uses the following features. Update accordingly.

• Unify ScInt & scValToBigInt into XdrLargeInt #809: not released yet
• Allow specifying mixed types for vectors in nativeToScVal #803