Support loading offline MDS blobs in MetadataService

[MasterKale]

This PR exports a new verifyMDSBlob() method from @simplewebauthn/server/helpers to enable FIDO MDS metadata statements to be validated and then cached, to be loaded later into a runtime that does not support runtime network egress. It accepts a single string value containing a FIDO MDS JWT, verifies the integrity of the JWT, and then extracts the metadata statements suitable for passing in as statements when calling MetadataService.initialize() later.

Fixes #732.

Example

On runtime that can make external network requests:

import { verifyMDSBlob } from '@simplewebauthn/server/helpers';

const blob: string = manuallyFetchMDSBlob();

// Makes network requests for things like CRL checks
const { statements, parsedNextUpdate, payload } = await verifyMDSBlob(blob);

await writeStatementsToDisk(statements);

On runtime that cannot make external network requests

import { MetadataService } from '@simplewebauthn/server';

const savedStatements = await readStatementsFromDisk();

// No network requests nor validation happen here because the statements are assumed trusted
await MetadataService.initialize({
  mdsServers: [],
  statements: savedStatements,
});