make $settings->idpList array optional or get rid of it altoghether

[simevo]

php-saml requires an Idp to generate the metadata (are we sure ?) so the class constructor needs at least one idp that can be set up as default; for this purpose we currently require the $settings->idpList array to have at least one entry

it should be possible to make either:

1. make it opitional: if the user does not provide an array, look in the idpMetadataFolderPath and load all xml files from there
2. or if we manage to generate the metadata w/o an IdP, use the alternative API where the constructor knows nothing about the IdPs (bonus: we can rename all keys from spXxxxYyy to xxxxYyy)

The IdPs would be configured separately, either loading all IdP metadata from a specified dir at once:

$sp->loadIdpMetadata(./'idp_metadata');

or loading only selected IdPs:

$sp->loadIdpFromFile('/srv/spid-myservice/idp_metadata/testenv2.xml'); // 0 = Test IDP
$sp->loadIdpFromFile('/srv/spid-myservice/idp_metadata/idp_1.xml');    // 1 = Infocert ID
// $sp->loadIdpFromFile('/srv/spid-myservice/idp_metadata/idp_2.xml');    // 2 = Poste ID
// $sp->loadIdpFromFile('/srv/spid-myservice/idp_metadata/idp_3.xml');    // 3 = Tim ID
// $sp->loadIdpFromFile('/srv/spid-myservice/idp_metadata/idp_4.xml');    // 4 = Sielte ID
// $sp->loadIdpFromFile('/srv/spid-myservice/idp_metadata/idp_5.xml');    // 5 = Aruba ID
// $sp->loadIdpFromFile('/srv/spid-myservice/idp_metadata/idp_6.xml');    // 6 = Namirial ID
// $sp->loadIdpFromFile('/srv/spid-myservice/idp_metadata/idp_7.xml');    // 7 = SPIDItalia Register.it
// $sp->loadIdpFromFile('/srv/spid-myservice/idp_metadata/idp_8.xml');    // 8 = Intesa ID