Have fallbacks for retrieving providers list

[CasperWA]

Heroku is currently failing, since providers.optimade.org is down.

I suggest we implement the following fallbacks (in order):

1. Try providers.optimade.org.
2. Try Materials-Consortia/providers on GitHub (raw providers.json file).
3. Go through providers submodule, if available.
4. Log warning, don't include providers.

Is this actually the original reason we include providers as a submodule? If yes, then I suggest to remove it as a submodule, removing also option 3 from above, since I don't consider the addition of the list of providers crucial if the list is not guaranteed to be updated. But that's my personal opinion :)

[CasperWA]

Reducing the urgency of this issue, since the SSL issues with providers.optimade.org seems to have been fixed. See Materials-Consortia/providers#45.

[CasperWA]

Concerning this, I actually think we should remove the providers submodule and simply try for first providers.optimade.org, then GitHub, and if nothing is returned, or the returned providers cannot be cast to the Provider pydantic model, then simply create a warning in the response?
I don't consider it essential to a successful response that the list of providers is included in the /links-endpoint. But it's worth warning about, since it should be included. Especially in this example server.

What do you think @shyamd and @ml-evs?

[shyamd]

The original reason was to be able to better track changes in providers and have tests run on those changes so we can see what broke. I have no problem not even getting data from the providers sub-module, but I'd like to keep it for dependabot. It's already saved a lot of headache once.