add ICompress interface

[dabutvin]

laying the groundwork from the discussion in #467

This doesn't change any behavior, only sets up the framework

We can open new issues with the following template to support any number of arbitrary libraries/algorithms moving forward.

new file called SomethingCompress.cs and implement the 3 functions

namespace CompressImagesFunction.Compressors
{
    public class SomethingCompress : ICompress
    {
        public string[] SupportedExtensions => new[] { ".XXX", ".YYY" };

        public void LosslessCompress(string path)
        {
            throw new System.NotImplementedException();
        }

        public void LossyCompress(string path)
        {
            throw new System.NotImplementedException();
        }
    }
}

Register the new compressor so we start picking it up at the top of CompressImages.cs

Update the Dockerfile.CompressImages file to properly install the library on the image