Handle providers that do publish SHA signature files
Several Hashicorp providers do not appear to supply signature files in their releases (e.g. https://github.com/hashicorp/terraform-provider-awscc/releases/tag/v0.77.0). This should be handled - maybe a flag on the provider (perhaps a global configuration whether to allow this or not?)
Github reference: https://github.com/MatthewJohn/terrareg/issues/47