saml2: Add GetServiceProviderMetaDataCmd that returns SP metadata XML

This adds GetServiceProviderMetaDataCmd which returns SP metadata XML, since
this information should be public for IdPs to discover, we implement this as a
login/cmd api so this does not require any kind of authentication to GET this

Signed-off-by: Rohit Yadav <rohit.yadav@shapeblue.com>
client: add getSPMetadata API in commands.properties

Signed-off-by: Rohit Yadav <rohit.yadav@shapeblue.com>
utils: fix SAMLUtils's authnrequest maker

Signed-off-by: Rohit Yadav <rohit.yadav@shapeblue.com>