Allow custom environment variable prefixes in settings provider

[jeremiahrose]

Following on from the discourse conversation here.

When using the settings provider source, it currently isn't possible to assign a prefix to an environment variable name without also having the prefix in the internal setting name. We have written a patch and it seems straightforward enough to allow an optional prefix to be configured in the register_provider block like so:

Application.register_provider(:settings, from: :dry_system) do
  prefix { 'SOME_PREFIX_' }

  settings do
    setting :database_url, constructor: Types::String.constrained(filled: true)
end

I would like to submit a PR for this soon, if maintainers feel it would be helpful!

@solnic also mentioned that:

Having ability to have nested keys from env would be great too like STRIPE__API_KEY would be turned into settings.stripe.api_key.

I agree that would be a nice feature, so I would be happy to implement that too.

[jeremiahrose]

Sorry about bug label, I can't seem to remove it. :(

[alassek]

I would have expected the prefix option to be passed in via register_provider kwarg, I am guessing that you chose a provider method so that the implementation would be limited to the Settings provider?

I wonder if it would be helpful to have some sort of options pass-through from the register_provider method into the provider initialization.

[jeremiahrose]

@alassek we went for a configuration option instead, PR up here: #258