Use JWT header alg when verifying MDS blob

[MasterKale]

This PR fixes MDS blob verification to properly use the JWT header's "alg" property to determine which hash algorithm to use to verify the signature.

MDS blobs are too big to include as test data (10MB+) so I verified locally that this fixed the issue identified in #786 with the latest MDS blob from https://mds3.fidoalliance.org that has "alg": "RS256" in its header:

import { verifyMDSBlob } from './src/helpers/index.ts';

const blob = await Deno.readTextFile('./blob.jwt');

const verified = await verifyMDSBlob(blob);

console.log(verified.parsedNextUpdate);
// 2026-09-01T07:00:00.000Z

Fixes #786.