Make `DatabaseLoginsStorage` async

[bendk]

There hasn't been agreement on a grand vision for async Rust in app-services (#6549). I think part of the reason is that ADR is discussing too many things. This is my attempt to take a small change from that ADR and see if we agree.

This change takes the async wrapping code that's currently in android-components and moves it to our app-services wrapper. This improves the API boundary IMO. The SYNC sync and the credentials management teams are responsible for threading issues. For example, we're the one's thinking through how to avoid blocking too many threads when we're waiting on the primary password dialog. Since we're responsible for threading issues, we should own the code

On a practical level: I have some ideas on how to improve our threading strategy and it's simpler to think about those changes if all the code is in one repo.

Here's the corresponding android-components change: https://github.com/bendk/firefox/commit/d038ba6fd1ec1b39f76e0720e594916d61a4600a

If we agree on this, then I think we can do something similar for iOS.

What do others think? Is this a good idea? Do we need/want an ADR for this?

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