Add support to provide private key as string in addition to file path

[chris1312]

Currently, the only supported source for private key is local file system.
See class \SAML2\Configuration\PrivateKey

If I want to store my private key in any other storage, I am forced to rewrite many other classes of saml2 library in order to utilize \SAML2\Response\Processor which is the best way to handle SAML responses.

Therefore, \SAML2\Configuration\PrivateKey class could also accept private key contents.