Add Microsoft provider

[rafaelmardojai]

This provider is implemented to test our needs when it comes to this kind of "official" APIs with private keys. Some open questions:

• We want to have some UI to show API consumption limit?
• Provide a way to expose a short description on how to get an API key?
• Is fine to store the API key in plain GSettings?

Blockers:

• [ ] In some cases Microsoft requieres to pass the "Subscription-Region" string, our current API doesn't allow providers to expose this kind of "extra config fields"

[mufeedali]

We want to have some UI to show API consumption limit?

Yes, I think showing this only in the preferences window would be fine.

Provide a way to expose a short description on how to get an API key?

I guess it would be a nice to have. But design consideration for this is important since we don't want to crowd the UI.

Is fine to store the API key in plain GSettings?

We need to switch to libsecret for the API keys alone I think.

[mufeedali]

Since we now have bing, let's hold off on this one.