[fix] Try deser with Base64 first

[joyqvq]

sui-sdk-types::UserSignature cannot deserialize from base64 string format, breaking compatibility with Sui's GenericSignature format https://github.com/MystenLabs/sui/blob/main/crates/sui-types/src/signature.rs#L304

e.g. client sends
{
"user": "0x123",
"signature": "ANp47jDZQszLl6ocHM..." // base64 encoded user sig
}

without the fix deser fails - UserSignature only accepts structured json like:
{
"signature": {
"scheme": "ed25519",
"signature": "2njuMNlCzMuXqhwcz9...",
"public_key": "DX2rNYyNrapO+gBJp..."
}
}

this change fixes it by accept both formats using an untagged enum, test added! (otherwise i had to implement a workaround: https://github.com/MystenLabs/seal/pull/412/changes#diff-7e3a1314e53d356a05b0214b65850dd9de61b41131fc5d4e3076fed0c3d450d4R55)