[all] Add Linux and Windows support

[cbenhagen]

Options are:

• C++ SDK
• Writing / Adopting existing dart-native implementations like firedart.

The second option might be harder but in the long run it might be easier to maintain.

(Related to #2029)

[stuartmorgan-g]

• C++ SDK

Note the following, from the documentation for that SDK:

Caution: Firebase C++ SDK desktop support is a beta feature. This feature is intended only for workflows during the development of your game, not for publicly shipping code.

I.e., it's an SDK for testing mobile games on desktop, rather than a desktop SDK.

[cbenhagen]

I guess pure Dart implementations would be the way to go then.

[salihgueler]

If we decide(e.g. I am planning to work on this) to tackle this, should we create separate issues per product and work on them accordingly?

[cbenhagen]

@salihgueler I think one meta issue and discussion in the PR would work as well but It's not up to me to decide. How do you plan to implement it?

[nilsreichardt]

An advantage of creating multiple tickets is that we can track which firebase package is how often requested.

[shivamdev-lgtm]

Is there any work being done on desktop support (other than MacOS)?