Extend SigningCallback form to allow returning an object

[Shaptic]

Closes #780 per discussion in #772 (comment): authorizeEntry can now take a callback that returns an object specifying the signer of the signature so that it isn't implied that it's the address in the entry. Namely, it can return

interface SigningCallbackResult {
  signature: Buffer | Uint8Array;
  publicKey: string;
}

[github-actions]

Size Change: +2.75 kB (+0.08%)

Total Size: 3.26 MB

Filename	Size	Change
dist/stellar-base.js	2.39 MB	+2.19 kB (+0.09%)
dist/stellar-base.min.js	869 kB	+561 B (+0.06%)

_{compressed-size-action}

[overcat]

LGTM!