Support email address(es) for Users

[jmpesp]

SAML assertions will most likely send the email address in the subject portion:

    <saml:Subject>
      <saml:NameID Format="urn:oasis:names:tc:SAML:1.1:nameid-format:emailAddress">some@customer.com</saml:NameID>
      ...
    </saml:Subject>

Whether or not this is pushed in via the Users POST, core should support storing it - it would be necessary to match against any SAML assertion.

[jmpesp]

Note this may not be necessary if externalId can be correlated to the SAML assertion's subject. Testing is required.

[jmpesp]

This is not required (yet), as Okta sends an assertion with the username

<saml2:NameID Format="urn:oasis:names:tc:SAML:1.1:nameid-format:emailAddress">michael+okta1@oxidecomputer.com</saml2:NameID>

[jmpesp]

Confusingly, with the name id format of emailAddress