Resolve "API for protected environments"

What does this MR do?

First part of #30595 (closed)

Implements get protected_environments/ and protected_environments/environment API calls. API is very similar to the protected_branches API we already have

I want to split it in 3 parts:

1. This MR
2. Post/Put/Delete methods
3. Documentation

API is hidden behind the feature flag to allow us change something in later without braking compatibility. I'll remove feature flag with the documentation MR.

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

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