Add Options class to backup cli

What does this MR do and why?

This MR is part of the effort to decouple the gitlab-backup-cli gem from the gitlab rails code: #454906

This MR adds an Options class to the gitlab-backup-cli gem to set values based on options passed to the cli and their default values. This class will be used in the next MRs that add additional targets to backup: !164205 (merged) and !163584 (merged)

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