Fix token for repository tree keyset pagination

What does this MR do and why?

Contributes to #377591 (closed)

Problem

The page token generation was done on Rails side before. It used a last element of the collection as a token.

It works for branches and tags, but the tree impementation requires to provide a token from GetTreeEntriesResponse.

Solution

Fetch the cursor from the GetTreeEntriesResponse and correctly assign it to Links header.

Refactor branches and tags finders to have a similar interface as TreeFinder.

Screenshots or screen recordings

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Before	After

How to set up and validate locally

1. Enable keyset pagination for tree Feature.enable(:repository_tree_gitaly_pagination)
2. Find a repository with more than 20 elements (files/directories) in the root directory
3. Make a call to repository tree API (adjust project id) http://127.0.0.1:3000/api/v4/projects/2/repository/tree?pagination=keyset
4. You should see a link header in the response with a page_token
5. Follow the link (something like http://127.0.0.1:3000/api/v4/projects/2/repository/tree?id=2&page=1&page_token=eyJmaWxlX25hbWUiOiJnaXRsYWItZ3JhY2sifQ%3D%3D&pagination=keyset&per_page=20&recursive=false)
6. You should see the second page of the response

MR acceptance checklist

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