SCIM GET /Users should support requests without a filter

What

Support SCIM requests to GET /Users that don't have a filter specified.

This was unblocked by SCIM pagination (#13094 (closed)) and the refactor for this overlaps with the supporting the eq filter in !15685 (closed).

Why

SCIM clients will sometimes send a request without a filter, using pagination to limit results instead. Currently we raise an error when a filter isn't provided, meaning we can't process those requests.

Filtering is an optional part of the SCIM spec, so we are required to support requests that do not have a filter. This will help increase the number of SCIM providers we support, and reduce the chance that our Azure integration breaks.

Bonus: this will help Group admins to query externalId values for troubleshooting, so will help with #33378 (closed)