Change SignatureMethod to be asynchronous

[xvrh]

This pull request change the signature of SignatureMethod callback to accept returning a Future

A lot of packages implements the crypto algorithms with an asynchronous API. I would like to implement the RSA-SHA1 signature method with webcrypto like this:

import 'package:webcrypto/webcrypto.dart';

void main() {
  final rsaSha1 = SignatureMethod('RSA-SHA1', (String key, String text) async {
    var privateKey =
        await RsassaPkcs1V15PrivateKey.importPkcs8Key(_privateKey, Hash.sha1);
    return base64Encode(await privateKey.signBytes(text.codeUnits));
  });
}