Introduce alert mapping builder feature flag

What does this MR do?

Introduces development feature flag multiple_http_integrations_custom_mapping and adds Custom mapping section to new alerts settings form - Mapping Builder itself will be introduced in consequent MRs

the section is behind 2 feature flag s http_integrations_list && multiple_http_integrations_custom_mapping

Plan for the feature rollout on frontend side

header	header
Introduce feature flag, add placeholder for Mapping Builder	workflowin review
Add mapping builder form with mocked data	❌
Add linking arrow	❌
Integrate GitLab mapping fields with backend	❌
Integrate parsing alert payload with backend	❌
Send test alert - integration with backend	❌
Save form - integration with backend	❌
Remove feature flag	❌

Screenshots (strongly suggested)

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

Related to #262707 (closed)