Implement pagination & filtering for group imports

What does this MR do?

This MR adds three things on frontend side:

• Displaying how many repositories are currently available for import

Related issue: #290237 (closed)

• Basic pagination

Related issue: #298798 (closed)

• Filtering

Related issue: #296957 (closed)

To support this on backend we are adding proper forwarding of pagination headers received from remote gitlab instance

Video

New_Group___GitLab__2_

Testing guidelines

• Open "New group" (/groups/new)
• Use https://gitlab.com as source instance and GeK1Nis4j-SY1X4sqE5c as personal access token (this token is from separate user on GitLab instance with 0 real data available, so we do not expose any security risks here)

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