Message Signature Validation

[Pivotal Issue 78632442][1]

[1]: https://www.pivotaltracker.com/n/projects/1163646/stories/78632442
[Here][1] two distinct configurations (at least, in parameter name) are injected. The IdP variant is [passed][2] to the [`checkSign`][3] method, but from there it can be any metadata configuration as per the docblock and parameter naming. Since the ported functionality should be backwards compatible with the SSP implementation, we should support the same.

My knowledge is too limited to be sure that there ever only will be one variant of metadata configuration will be used here. With the separation of configuration between IdP and SP, they needed a shared identifier, hence the marker interface. It is still on my todo-list to discuss this behavior with you, but wanted to hold that off till at least everything was functionally ported :wink: 
In any case, the question is: can indeed both SP and IdP configurations be given here, or is my hunch that only IdP configuration may be used correct (in which case I remove this interface and typehint against the IdP conf object).

The possible behavior that I wanted to expose is to expose distinct getters for the `keys`, `certData` and `certificate` values without having to rely on SSP dictated string keys that are not communicated anywhere, and which documentation I have not found yet (albeit I have hardly searched :tongue: )

[1]: https://github.com/simplesamlphp/simplesamlphp/blob/master/modules/saml/lib/Message.php#L486
[2]: https://github.com/simplesamlphp/simplesamlphp/blob/master/modules/saml/lib/Message.php#L503
[3]: https://github.com/simplesamlphp/simplesamlphp/blob/master/modules/saml/lib/Message.php#L136