Add keys.name and key_name configuration options

lib/SimpleSAML/Configuration.php

lib/SimpleSAML/Metadata/SAMLBuilder.php

lib/SimpleSAML/Utils/Crypto.php

modules/saml/docs/sp.md

modules/saml/lib/Auth/Source/SP.php

modules/saml/lib/IdP/SAML2.php

tests/lib/SimpleSAML/Metadata/SAMLBuilderTest.php

...ib/SimpleSAML/Metadata/test-metadata/www.example.com.cert

...lib/SimpleSAML/Metadata/test-metadata/www.example.com.key

...impleSAML/Metadata/test-metadata/www.example.com_new.cert

...SimpleSAML/Metadata/test-metadata/www.example.com_new.key