Add Add-on providers

Recently we had an issue were TuSubtitulo (https://github.com/morpheus65535/bazarr/issues/1591) requested their provider to be taken down. If all providers do the same and we comply with it no providers will be left. The project has a lot of potential but it should be a way to delegate responsibility for the providers development. I don't know if currently there is a way but as a developer I propose the following:

+ Develop a simple framework were we can plug-in our own providers.
+ Providers should be refactored into independent Github projects. This will allow updating providers without updating Bazarr as a whole.
+ Allow (with an SDK or a simple webhook) to create our own providers easily.

I think this is an important feature and probably the majority of the users won't use it but it's crucial for the future of Bazarr.

Thanks