Some things, like logging of SAML messages or backtraces, are controlled with the 'debug' configuration option. However, it might be possible that we don't want one while we want the other, but that's impossible with just one option.

This commit allows us to configure debugging options independently, but groupping all of them together. This is particularly useful if we want to log backtraces to debug errors, for example, but we don't want to log SAML messages to keep the privacy of the users. This also allows us to get rid of the 'debug.validatexml' configuration option, and group it with other debug options.

This changes are backwards-compatible. Old and new configurations will work at the same time.

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.

Use InvalidArgumentException instead of SimpleSAML_Error_Exception when dealing with wrong input parameters.

Move SimpleSAML_Utilities:: debugMessage() to SimpleSAML\Utils\HTTP::debugSAMLMessage() and deprecate the former.

Move SimpleSAML_Utilities::getDOMChildren() to SimpleSAML\Utils\XML::getDOMChildren(). Deprecate the former.

Move SimpleSAML_Utilities::isDOMElementOfType() to SimpleSAML\Utils\XML::isDOMElementOfType(). Deprecate the former.

Move SimpleSAML_Utilities::getDOMText() to SimpleSAML\Utils\XML::getDOMText(). Deprecate the former.

Move SimpleSAML_Utilities::formatXMLString() to SimpleSAML\Utils\XML::formatXMLString(). Deprecate the former.

Move SimpleSAML_Utilities::formatDOMElement() to SimpleSAML\Utils\XML::formatDOMElement(). Deprecate the former.