Add create and delete methods for protected environments API

What does this MR do?

Related to #30595 (closed)

It's split in 3 parts:

1. get methods: !22336 (merged)
2. this MR, adding create/delete methods
3. documentation, feature flag removal

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