Support base64 in validator domain verifier tool

[mDuo13]

The manifest method provides a relatively easy¹ way to look up a validator's manifest based on its public key, but it only provides the response in base64. Meanwhile, the Domain Verifier Tool only accepts manifests in hex.

It can work if you separately convert the base64 to hex, but that's something that can be done in-browser, so it would make sense to update the Domain Verifier Tool to accept base64 directly, and maybe even add an option to look up domain verification strictly by pubkey (this would need to connect to a public p2p node & call manifest before doing the rest of the work the checker already does).

¹note: s1's reporting mode servers don't support manifest, but xrplcluster's p2p servers do