Include external users in user search

What does this MR do?

The request GET /api/v4/users returns different responses for normal users and admin users so far:

• For admin user: returns all users (including external users)
• For normal user: returns only all non-external users

Since there's no known reason for excluding external users by default for requests from normal users, the following changes are done in this MR:

• By default, the request GET /api/v4/users returns all users (including external users) for normal users as well.
• Similar to the existing exclude_internal=true parameter for excluding internal users, a new parameter exclude_external=true has been implemented. This parameter can be used to exclude external users if desired.

Solves: #263128 (closed)

🛠 with ❤ at Siemens

/cc @bufferoverflow

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