Allow custom providers in provider() type

[dw-kihara]

aws_credentials allows users to add custom providers.
However, the aws_credentials_provider:provider() type is currently limited to built-in module names
causing custom providers to be rejected by Dialyzer because their names are not listed in the provider() type.
Example:

C = aws_credentials:make_map(my_provider_module, AccessKeyId, SecretAccessKey, Token),
                          %% ^ This atom is not in the `provider()' type, so Dialyzer rejects it.

This pull request adds the module() type to the provider() type, allowing custom providers to pass Dialyzer checks.
Additionally, I made some minor changes related to custom providers.

[dw-kihara]

The module() type is effectively the same as the atom() type. However, I think this change clarifies that it represents a module name to the user.

[onno-vos-dev]

Guessing this is related to your work in: #73 correct? While you added an explicit new type there I'm fine with this change 👌 I would however encourage you to keep the explicit type in the other branch (#73) when you rebase that one 👍

[dw-kihara]

Guessing this is related to your work in: #73 correct?

Yes! I had trouble when writing that provider as a custom provider, so I made this pull request.

I would however encourage you to keep the explicit type in the other branch (#73) when you rebase that one 👍

Agree, I also think the default providers should be listed there.