Add GitHub Importer pagination

What does this MR do?

This MR adds pagination capability (infinite scrolling) to GitHub Importer. Previously, when remove_legacy_github_client feature flag is enabled, it would concatenate all pages together and presents all repos to the user. Now, we return just the specified page.

Few notes:

1. Codepath when remove_legacy_github_client is off does not support pagination and it going to be removed in the future MR
2. Search API and regular repositories API is different and 2 separate endpoints have to be used. Hence there is a different codepath between when filter param is present or not

Mentions #27370 (closed)

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