[Connector/SAML] Support metadata xml discovery

[kenchan0130]

Overview

Some values are now automatically retrieved from IdP metadata URLs.

What this PR does / why we need it

In SAML connector, the implementation of the metadata discovery function remained a TODO in source code.

Special notes for your reviewer

There are negative opinions about SAML with regard to maintenance.

Therefore, we have reduced the maintenance load by using third-party libraries instead of creating our own logic.

Does this PR introduce a user-facing change?

A new config option for SAML connector. This is not a breaking change.

In https://dexidp.io/docs/connectors/saml/#configuration, idpMetadataURL attribute must be optionally added to the config value.