Return a pagination cursor from `FindAllTagsRequest`

Similar issue for branches: Return a pagination cursor from `FindLocalBranc... (#431903)

Problem

FindAllTags RPC already supports pagination parameters.

But it doesn't respond with a next pagination cursor, as GetTreeEntriesResponse for example.

Because of that the cursor generation logic is implemented twice: by Rails and by Gitaly.

Proposal

• Use Gitaly as a SSOT for pagination cursor generation.
• Extend FindAllTagsResponse to return PaginationCursor.
• Update Rails code to use Gitaly cursor instead of building it manually.