Parse IDP properties from metadata

[jgomer2001]

To register a SAML provider, some properties data such as cert, logoutUrl, entryPoint, identifierFormat might be grabbed from IDP's metadata.

This enhancement can make the configuration process more agile.

[jgomer2001]

Additionally, if we fetch IDP's metadata URL with certain frequency, we can update cert automatically and solve #59

[jgomer2001]

The process requires human intervention as well, for instance, entryPoint depends on a previously selected binding. Bindings offered are grabbed from metadata.

About the same goes for identifierFormat.

Since oxtrust does not necessarily have access to Internet, we still have to allow admins to enter the values directly in the forms (or offer a metadata upload facility).

[christian-hawk]

I understand this should be a nice feature indeed.
As discussed in support ticket 8984:

Fetch what we can from metadata, ask user to configure what's missing