[WIP] Add async settings loading and Jetpack datastore support

[pelmenstar1]

• Async setting loading means that the operation can take some time and result in an error. So I added load-states (Loading, Error, and Success).
• Also settings are loaded in snapshots because it would be strange if a single setting had its own load state. Besides, it's easier to track the state when snapshots are used.

To add:

• Screens in demo
• Controls support
• Loading indicator
• Error handling

[pelmenstar1]

I haven't decided which version of key creation is better to use (one with dataStore*type*settingsSnapshotKey or dataStoreSnapshotKey builder). @alorma What do you think?

[alorma]

Hi @pelmenstar1

Beside I love the Idea of the async preference, I'm not sure on merging it also with the data store implementation.

So, I've decided to check it and got an approach to use datastore in a synchronous way, I've made the work here:

#45

Could you check it and give me your thought?

That's just a first implementation, later on it could came up with your async solution. But I think it's a good approach for a first datastore impl.

[pelmenstar1]

@alorma Ok, no problem.

BTW, I don't think the use of Jetpack DataStore in a synchronous context is a good idea, because:

• the disk IO can be really slow
• no error handling

[pelmenstar1]

I'm closing the PR as the implementation of Jetpack DataStore support, which better fits in the current state of the library, has been merged.