[CILogon] Add idp config allowed_domains_claim for use with allowed_domains

[consideRatio]

An opt-in non-breaking step along the way to address #547. The issue is perhaps best solved long term by doing the breaking change of letting allowed_domains_claim default to "email".

Change summary

• (unrelated bugfix) Makes allowed_domains no longer be case sensitive, and adds test case
• Adds allowed_domains_claim config (docs updated, schema updated, a test is added)
• Makes the specified claim able to directly specify a domain name, and not only indirectly via an email like string.