Add configuration option to specify relying party id

[bnf]

WebAuthn credentials are always bound to a specfic relying party id (RP-ID) (=domain).
This relying party id is defined during the attestation dance (registration phase).
bnf/mfa_webauthn currently uses HOST_NAME as relying party id.
This is a key WebAuthn feature to prevent credential spoofing via faked domain names.

Usecase for configurable RP-IDs:

If the TYPO3 instance is available via https://*.mydomain.com, the webauthn spec allows the relying party id to either be instance.mydomain.com or mydomain.com.
The former will only allow credential-verification via instance.mydomain.com, the latter will allow verification via mydomain.com and all its subdomains.

[bnf]

We should ideally derive the value from https://docs.typo3.org/m/typo3/reference-coreapi/main/en-us/Configuration/Typo3ConfVars/SYS.html#confval-globals-typo3-conf-vars-sys-cookiedomain – if configured.