Feature flag: Allow SCIM API to return SAML identity

What does this MR do?

Related to #217509 (closed)

Prior to separating SCIM and SAML identities, the SCIM API could be used to get SAML-only users. Some customers came to rely on this for auditing purposes. Despite it not being an intended use of the API, we're reintroducing the functionality temporarily while we address some additional deficiencies in SCIM support. This feature is behind a feature flag and will only be enabled for customers with a broken workflow.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team