Add project files.

Beam.Exports/IStreamExporter.cs

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+namespace Beam.Exports {
+    public interface IStreamExporter : IAsyncExporter {
+        /// <summary>
+        /// Asynchronously writes the object to the desired path in many parts, returning the path
+        /// of each written file as a stream
+        /// </summary>
+        /// <param name="path">The path of the exported object</param>
+        /// <returns>The async enumerator of each written file</returns>
+        public IAsyncEnumerator<string> WriteAsyncStream(string path);
+    }
+}