SCIM GET /Users supports requests without a filter

What

Adds support for SCIM requests that do not have a filter.

Closes #35209 (closed)

Works towards #13138 (closed)

Why

SCIM clients will sometimes send a request without a filter, using pagination to limit results instead. Previously we would raise an error when a filter wasn't provided, meaning we were unable to process those requests.

This is needed for Okta compatibility, since their setup first tests SCIM with startIndex=1&count=1 with no filter.

Screenshots

Risks

• Changes to SCIM risk breaking user provisioning, but here that is mitigated by only changing GET /Users rather than the main process of creating and updating users. Verifying this change against an Azure setup would further mitigate this risk.
• We'll want to verify behaviour on Okta before updating docs to mention any support. In particular we don't yet follow the SCIM spec regarding IDs, and there is a risk that could lead to duplicate users and users unable to sign in so we will not treat this MR as having implemented Okta support.

Acceptance criteria

• Change target branch back to master after !19417 (merged) merges
• Changelog entry
• Documentation created/updated or follow-up review issue created
• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.