WIP: Basic SCIM eq filter support

What

• Adds support for SCIM requests that do not have a filter
• Adds support for SCIM requests with basic 'eq' filtering instead of hard-coding an external_uid lookup.

Why

• Some SCIM requests do not have a filter and instead rely on pagination
• Some SCIM requests expect to be able to choose which field to filter by and return multiple results

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/13138

Todo

• Actually implement the above 😉
• ScimFinder should support eq filtering
• Scim::ParamsParser processes params based on request instead of guessing by contents.

Screenshot / Example

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

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