Refactor: Integrations require enumeration of providers inside the library

[toddb]

Providers are currently stringly known and undocumented.

The full list is here and should be introduced into the library.

However, C# doesn't implement enums with inheritance that you get with typescript. Either

• implement a global provider list and attribute provider types (aka tag)
• implement an enum-type factor provider

Note: validators could be put on the models and/or auto population of provider type based on name.

[wh1337]

With this, we would have to keep this in sync with Novu to ensure downstream clients aren't effected. Not impossible or alot to do, but just wanted to make the statement