support OpenID Connect

[simevo]

the library is in principle ready to support flows alternative to SAML (see https://github.com/italia/spid-php-lib/blob/master/src/Sp.php#L21)

[lussoluca]

Some work has been done here: https://github.com/italia/spid-cie-oidc-php/tree/main