Add guide for writing a `Provider`? about dotslash

[alilleybrinker]

Totally makes sense regarding not wanting to commit to the Provider design at the moment. Perhaps then it doesn't make sense to document how to implement a Provider (the design today is also simple enough that a reasonably capable Rust programmer should in theory be able to figure out what's needed with relative ease). So long as the process of adding one stays the same, you could also always point people to this issue and to the draft PR for a GitLab Releases provider to show what's necessary.

There's also the fact that so far the docs for Dotslash appear to be focused on users of it, not contributors to it, so docs on how to write a Provider would be a little out of place there.

Thanks! Happy to consider this issue resolved for now then.

[alilleybrinker]

(Disclosure: my interest here is that it would be nice to add a GitLab provider which works similarly to the GitHub provider, downloading released artifacts built by CI from a GitLab instance)

[bolinfest]

From the current docs:

(At the time of this writing, there is no way to add custom providers without forking DotSlash.)

Relatedly, I don't think that we are ready to commit to the contract of Provider just yet. To be clear, I am open to considering #3, but it would have to be a built-in provider for now. That way, any changes to the Provider trait could be made atomically with their implementations.

My only question to validate at the moment is the proper handling of the provider config. It looks like the intent is that the provider config is given as a valid "loosely parsed" JSON value, which the provider itself is then able to deserialize into any specific structure it likes. Do I have that right?

Yes, that is correct.

[bolinfest]

@alilleybrinker FYI, I just put up an RFC for this: #7.