Add TabsStorage to concept-storage

[jonalmeida]

Adding an interface for TabsStorage to the concept-storage component, similar to the HistoryStorage.

This allows us to support the feature components, and also be a provider for the AwesomeBar.

┆Issue is synchronized with this Jira Task

[jonalmeida]

This should allow for storing open tabs and retrieving them. Probably identified to know which ones are device tabs vs. sync'd ones.

Not sure yet what that should look like.

[eoger]

Side-note: Synced Tabs might evolve to group tabs per-window in the future, maybe there's a way to accommodate for that?

[jonalmeida]

How would you identify a window in the sync data? Currently, we don't have a concept of a "window". The closest thing we'd have in Fenix are Collections and those are still quite detached from the rest of the tab paradigm (also undergoing user studies on their usefulness).