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!

[kalepail]

I don't feel like I have much to add here. What I'm working on doesn't involve signing with G-addresses much anymore so I wouldn't be using this. If I do sign with a G-address the entire auth flow is custom as per whatever the __check_auth signature arg determines. Which this definitely isn't supporting, but that isn't to say such a method isn't useful, just that I can't really speak to it.