Allow IdP to have separate config settings including field mappings

[danmarsden]

rebased version of #804

[danmarsden]

not ready for merging yet - currently reviewing and testing.

[danmarsden]

@sarahjcotton @jacdsouza - do you guys have any modified versions of this one? - it seems to be failing unit tests for me with errors like the following: (github actions isn't triggering here, but possibly due to the 5.0 branch being tested.)

1) auth_saml2\auth_test::test_update_custom_user_profile_fields with data set #0 ([['Test data']])
Failed asserting that false is true.
moodle/auth/saml2/tests/auth_test.php:1253
2) auth_saml2\auth_test::test_update_custom_user_profile_fields with data set #1 ([['A different string']])
Failed asserting that false is true.
/moodle/auth/saml2/tests/auth_test.php:1253
3) auth_saml2\auth_test::test_missing_user_custom_profile_fields with data set #0 ([['Test data']])
Failed asserting that false is true.
/moodle/auth/saml2/tests/auth_test.php:1300
4) auth_saml2\auth_test::test_missing_user_custom_profile_fields with data set #1 ([['A different string']])
Failed asserting that false is true.
/moodle/auth/saml2/tests/auth_test.php:1300
5) auth_saml2\auth_test::test_update_user_profile_fields_updates_username_on_creation
Failed asserting that false is true.
/moodle/auth/saml2/tests/auth_test.php:1411
6) auth_saml2\auth_test::test_update_user_profile_fields_updates_username_on_creation_case_insensitive
Failed asserting that false is true.
/moodle/auth/saml2/tests/auth_test.php:1437
7) auth_saml2\auth_test::test_update_user_profile_fields_updates_mapping_field_on_creation
Failed asserting that false is true.
/moodle/auth/saml2/tests/auth_test.php:1489
8) auth_saml2\auth_test::test_update_user_profile_fields_multi
Failed asserting that false is true.
/moodle/auth/saml2/tests/auth_test.php:1554