Refactor metadata_manager to move to versions provider based model

[shiv-tyagi]

This PR refactors the versions manager to move to a versions provider based model.

We introduce three providers:-

1. ManifestJson provider which parses manifest.json from the firmware server and list officially released versions on custom build server based on that. We are moving away from a release git tags based method, to make sure we are in sync with official firmware server and also make sure we have features.txt for all versions we have.
2. A whitelisted-tags provider which pulls tags in repo of whitelisted forks and list them for building on the custom build server.
3. remotes.json based provider to allow manually listing versions on custom build server by adding entries into the file.

I have tested this locally and it works good.

I will update #242 to use the same manifest json client we introduced in this PR to help listing the standard builds for a vehicle-version-release combination.

[peterbarker]

Introduces a race condition where the SHA a user is dealing with can disappear while they're playing in the interface. Should be explicitly commented on somewhere. Even better if we can find a neat solution :-)

LGTM

[shiv-tyagi]

Thanks for the review @peterbarker. I have addressed your comments.

Introduces a race condition where the SHA a user is dealing with can disappear while they're playing in the interface. Should be explicitly commented on somewhere. Even better if we can find a neat solution :-)

The race was always there. Even when we used github tags to fetch releases. It is very hard to know if there is anyone on the client side who has fetched a SHA but it moved before the hit build. I will think about reducing the possibility of that happening.