feat: Environment Variable Provider

[MJRichardson]

Environment Variable Provider

This PR adds a provider that evaluates environment variables.

It will use the flag key as the name of the environment variable, but also provides the ability for a common prefix to be supplied when the provider is created.

            // If no prefix is required, the default constructor can be used, e.g.
            // var provider = new EnvVarProvider();
            var provider = new EnvVarProvider("FeatureToggles_");
            await OpenFeature.Api.Instance.SetProviderAsync(provider);