Add a check that ensures the metadata does not contain the attributes 'cacheDuration' and 'validUntil'

[AntonioGiovanniSchiavone]

The attributes "cacheDuration" and "validUntil" are not provided for by the SPID technical rules.
While some IdPs ignore their presence, for others, the presence of these attributes leads to rejection of the metadata.
It would be advisable to insert a check to verify their absence.