PSR-12 compliancy

Some metadata publishers offer SHA256 fingerprints to validate the
fingerprint of the certificate used to sign metadata. Allow users to
configure the fingerprinting algorithm to facilitate validation.

For backward compatibility, the default algorithm remains SHA1.

Now we are finally using the 2.x branch of the SAML2 library, which was also migrated to use namespaces. Even though the library provides an autoloader that allows loading the classes with the old names using class aliasing, we need to do the migration in one commit (at least for most part of it). This is due to the way SimpleSAMLphp checks data types, using inheritance to check objects agains abstract or more general classes. Even though class aliasing works, there's no way to replicate those relationships, and type checks that use the old class names will fail because the aliases are virtually new classes that don't inherit from others.

- Add test case
- Fix .gitignore since it ignored all metadata folders