Get feature flag definitions via API

What does this MR do?

This is a preliminary work for #273167 (closed).

This is based on: !45904 (merged). It is done, but the !45904 (merged) should be merged first.

ChatOps change: gitlab-com/chatops!158 (closed).

It makes us

1. Load feature flag definitions in production (which is done at load time)
2. Makes us validate feature flag name used on POST => This will be a follow-up and should be merged after gitlab-com/chatops!158 (closed).
3. Return definition details on GET/POST

Why

1. This is to ensure that only a defined feature flags are used
2. Still allow to skip the check if we did not cover all feature flags.

Impact

1. Since no all type: :ops are tracked, for them it will require --force to be used
2. We will be able to show as part of ChatOps current and obsolete feature flags!!!

This misses

1. ChatOps change to add --force

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