Resolve "No additional integrations for Core users"

What does this MR do?

A small MVC to handle the #263357 (closed) where we limit the amount of possible integrations for GitLab Core users to 2, GitLab Premium users can add as many as they need(only a single PROMETHEUS integration is supported regardless).

This feature sits behind a feature flag: http_integrations_list for the frontend and multiple_http_integrations for the backend.

Design

This merge supports this Epic: &4442 (closed) and will come in the following stages:

Merge	You are here
Add initial wrapper for new form and feature flag	!45993 (merged)
Add GraphQL read endpoint to new form	!46212 (merged)
Build out HTTP and Prometheus steps for new form / Add GraphQL Add HTTP endpoint to new form	!46647 (merged)
Add GraphQL Update / Reset Token(Add GraphQL support for updating alert integrations) endpoint to new form	!46697 (merged)
Add GraphQL delete integration endpoints(backend here: !45843 (merged)	!46849 (merged)
Add ee support to add multiple endpoints	🤘
Opsgenie && Test Payload	TBA
Abstraction/clean up	TBA
Remove feature flag(UX review happens here to keep the cost of work low) and delete old form code / REST API code	TBA

Screenshots (strongly suggested)

Free slot for integration(only one added for a core user)	Max integrations added for core user

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 #255603 (closed)