Given a policy and AWS credentials, produce a S3PostUploadSignedPolicy.
            
 Declaration Syntax
 Declaration Syntax| C# | 
public static S3PostUploadSignedPolicy GetSignedPolicy( string policy, AWSCredentials credentials )
 Parameters
 Parameters- policy (String)
- JSON string representing the policy to sign
- credentials (AWSCredentials)
- Credentials to sign the policy with
 Return Value
 Return Value