Add support for automatic registration of workers and sub-workflows when registering a workflow

[ngrahovac-codaxy]

Instead of having to register workflows and all their tasks manually (error-prone for large workflows), it could be useful to add support for automatic worker and sub-workflow registration. The user would then register only the workflows they need, and all of its workers and sub-workflows would be discovered via reflection and registered behind the scenes.

This feature can be optional, so something like this:

builder.RegisterWorkflow<MyWorkflow>(opts => opts.RegisterTasksAutomatically());