feat: allow interaction handlers of any observable type

[kentcb]

Fixes #1260.

[coveralls]

Coverage increased (+0.01%) to 65.574% when pulling 50fe504 on kentcb:Issue1260 into baab571 on reactiveui:develop.

[ghuntley]

• Is this a breaking change and/or do we need to offer a migration path?
• Documentation will need to be updated once this is merged.

[ghuntley]

Should we keep public IDisposable RegisterHandler(Func<InteractionContext<TInput, TOutput>, IObservable<Unit>> handler) as an alias method to the new signature but annotate it with [Obsolete("Please use RegisterHandler<T> instead?")] as a migration path? I want to stop making hard public API contract changes. Your thoughts?

[kentcb]

@ghuntley it's not a breaking change in the sense that if people were accidentally calling the non-observable version because their observable was not Unit, this change will actually help them.

[ghuntley]

Alright. Can you please update the docs and once done, self-merge. Sometimes in our release notes we draft up something more verbose and human to explain the rational behind a new feature (i.e. advertise the benefits) - this change would be a good candidate.

[kentcb]

@ghuntley not sure which docs you mean? Nothing has changed from a consumer point of view apart from the API being more forgiving of the observable type. But the docs don't talk about that any way.

[ghuntley]

Apologies, an assumption on my part that documentation would need updating. If they don't, they don't. I've started thinking about making this a standard question when merging pull requests - does the doc need updating and if so please do it. I'm keen to merge the documentation repo back into the main RxUI repo at some stage this year so documentation gets versioned along side the code.