Use pagination to load epics in roadmap

What does this MR do?

Note: Changes of this MR are behind feature flag :performance_roadmap. See the roll-out issue #337198 (closed) to learn more.

Use pagination via IntersectionObserver to load epics in the roadmap on scroll instead of loading all epics at once, thus considerably reducing the time it takes for roadmap page to load initially by speeding up both; GraphQL fetch and roadmap app initialization. This MR sets page size to 50 but we can change it to any limit we want, although less is better.

How to test it locally?

Here are the steps;

1. You'd first need to have a group in GDK that has more than 50 epics. We have a script that can create epics in bulk, so follow the guide present in the script snippet.
2. Once you have over 50 epics, enable feature flag for the group by running Feature.enable(:performance_roadmap, Group.find_by_full_path('<your-group-full-path>')) in Rails console.
3. Visit Roadmap page for the group on GDK and it should load items with pagination.

Screencast

Roadmap

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
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• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• 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 #299814