Passkey plugin - Calls to passkey-server during message signing seem to be redundant

[hjornigur]

Normally there is a need to verify a signature on a backend when the challenge signed was issued on the backend as well. This is not the case with signing a user operation.

In fact, all of the things required to sign and send a user operation are already present in passkey validator.

Removing the calls to sign-* endpoints would greatly simplify integrations with custom passkey servers.

How to get all the necessary data without reaching out to the passkey-server? For the sign options:

• challenge can be derived from the formattedMessage by converting it to base64url format;
• allowCredentials can be formed using the credential ID which can be passed e.g. in the WebAuthnKey object.

For the encoded signature object:

• signature and authenticatorData can be extracted from cred.response object.