Add the ability to add provider warnings

[Yantrio]

Description of the feature you are looking for.

tofu understands that if the .Warnings field is populated when fetching metadata about a provider that it should display those warnings to the end user.

Right now we don't have a method to add a warning to a provider or a specific provider version in this registry. We should allow people to fill in a form for a whole provider, or just a provider version to add warnings that will be displayed when users try to install a provider.

[cube2222]

I'd prefer doing this in a non-freeform way if possible.

That is, introducing fields like "mark_as_bad": bool that we would then translate to standardized error messages when rendering.