Make saml_use_subject configurable

[jaredmoody]

I'm working on supporting multiple Idp's, and some need saml_use_subject to be true and some false. Would you be open to a PR that allows setting some adapter to make this setting dynamic per Idp?

[adamstegman]

Sure, that’s fine. A class is probably ideal, that would make it easier to test and refactor in the future.