Replace strings with more appropriate types

[nitzel]

• Replace string with Uri where possible and sensible
• Use an enum for the signature method to avoid this magic string, e.g.:

    public enum SignatureMethod
    {
        HMAC_SHA1,
        RSA_SHA1,
        PLAINTEXT
    }

    public static class ExtensionMethods
    {
        public static string MapToString(this SignatureMethod signatureMethod)
        {
            switch (signatureMethod)
            {
                case SignatureMethod.HMAC_SHA1:
                    return "HMAC-SHA1";
                case SignatureMethod.RSA_SHA1:
                    return "RSA-SHA1";
                case SignatureMethod.PLAINTEXT:
                    return "PLAINTEXT";
                default:
                    throw new ArgumentException(nameof(signatureMethod));
            }
        }
    }