Add abstraction for creating signed tags from tags

Jerry Setorequested to merge
tag-signature-read-through-cache-4 into tag-signature-read-through-cache-3

What does this MR do and why?

Related to: #570537

This change decouples SignedTag from Gitlab::Git::Tag by introducing abstraction for the specific attributes from the Tag that SignedTag depends on.

This is to support constructing SignedTag in a sidekiq worker in a future MR: !211110

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Edited by Jerry Seto

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