PSR-12 compliancy

* Allow list of entity ids to be loaded to better support idpList

Certain metadata sources, like mdq, were don't support loading all metadata which
prevented them from being used with idplist. This change adds support for allowing
metadata sources to load a list of entities, allowing MDQ to be used for idp list.

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.

* Support metadata XML directly in config
Also consider 'xml' as a key, in addition to 'url' and 'file'.
When provided, attempt to load this directly as a metadata XML string.
This is useful when working with dynamic configuration and external polling of metadata updates.

* Test loading XML from a fixed string
No preexisting test in this area
Adjust XML loader to only get global config in file mode - otherwise it is
unused, and it might pollute the test usage.
Test that an actully invalid config throws an exception, but that minimal valid
(albeit useless) XML is accepted.

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

* One test suite for all tests.
* One file per class that is tested.

These changes should make it easier to add new tests.