RESTClient: add support for relative next URLs in LinkPaginators

[burnash]

Description

This pull request:

• introduces handling of relative next URLs in classes derived from BaseNextUrlPaginator.
• improves mock_api_server()

Related PRs

• Add RESTClient #1141

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[VioletM]

LGTM!

As I said it also could be a case

{
                "next_reference": "/api/resource?page=2&size=100",
            }

But I guessed it could be handled separately or not handled at all