feat: Add Subject ID attribute support

[hf]

Add support for the SAML V2.0 Subject Identifier Attributes Profile Version 1.0 which supersedes persistent NameID.

Additionally, adds support for passing in a different NameIDFormat.

[solodyagin]

Fixes #441

[scottsitar-yeshid]

Hi @hf, we ran into this issue as well. Thanks for putting together this PR! In the meantime, we're using the workaround of having a custom AssertionMaker in our IdentityProvider that first calls DefaultAssertionMaker.MakeAssertion(), and then patches values we'd like to change before returning.