Given a policy and AWS credentials, produce a S3PostUploadSignedPolicy.
public S3PostUploadSignedPolicy GetSignedPolicy( String policy, AWSCredentials credentials )
JSON string representing the policy to sign
Credentials to sign the policy with
Supported in: 4.5, 4.0, 3.5
Supported Versions: 4.6 and above
Supported Platforms: Android, iOS, Standalone