Export Request Message Update to support Remote App Execution

[JoeBatt1989]

*Describe the solution you'd like
Workflow Manager must be able to attached plugins to an export request message so that MIG knows what to do with the message, plugins can be one or more types.

New ExportRequestMessage

public class ExportRequestEvent : EventBase
{
        ...
        /// <summary>
        /// A list of data output plug-in type names to be executed by the export services.
        /// Each string must be a fully-qualified type name.
        /// E.g. <code>MyCompnay.MyProject.MyNamepsace.MyPlugin, MyCompnay.MyProject.MyNamepsace</code> where
        /// <code>MyCompnay.MyProject.MyNamepsace</code> is the name of the assembly (DLL).
        /// </summary>
        public List<string> PluginAssemblies { get; private set; }
        ...

[JoeBatt1989]

@mocsharp @DanielNuttall-AD @neildsouth we need to define what the schema is for the new ExportRequestMessage and how WorkflowManager would know what plugins are available, do we need a lightweight API request to MIG to make it extendable? Obviosuly could be hardcoded for now but would be good to have a single source of truth for the plugins we support

[mocsharp]

@DanielNuttall-AD @neildsouth @JoeBatt1989
I have updated the MR Project-MONAI/monai-deploy-messaging#210 to include a list of plug-in assembly names instead of using an enum. Please review.

[neildsouth]

The above looks good and makes sense, I think if the MIG had an endpoint to query the available export plugins as @JoeBatt1989 suggested, that returned a list including fully qualified name, displayable name and description, that would complete the loop and give us info we could display to a user for selection?

[mocsharp]

I will create another task to provide the APIs to query for input/output plug-ins.