Add `alert_runbooks` feature flag

What does this MR do?

Adds a feature flag for the Alert runbooks feature, to aid development.

The flag will be named alert_runbooks.

Also adds an empty field to the frontend and enabled/disabled specs for the flag using this field.

• Feature issue: #208490 (closed)
• Feature flag rollout issue: #232462 (closed)

Screenshots

before	after (alert_runbooks disabled)	after (alert_runbooks enabled)

Does this MR meet the acceptance criteria?

Conformity

• [n/a] Changelog entry
• [n/a] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [n/a] Database guides
• [n/a] 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.
• [n/a] Tested in all supported browsers
• [n/a] 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:

• [n/a] Label as security and @ mention @gitlab-com/gl-security/appsec
• [n/a] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [n/a] Security reports checked/validated by a reviewer from the AppSec team