Allow building workflows as typed sequences

[glopesdev]

To support more flexible programmatic manipulation of expression builder workflows, this PR introduces a new overload to compile an expression builder graph into an observable sequence where the type of the elements in the sequence is known.

This makes it possible to load a workflow and perform a type-safe subscription to access the results of the compiled observable sequence, as shown in the example below:

using var reader = XmlReader.Create(path);
var workflowBuilder = (WorkflowBuilder)WorkflowBuilder.Serializer.Deserialize(reader);
var observable = workflowBuilder.Workflow.BuildObservable<object>();

Both covariance and explicit type conversions are allowed in case of type mismatch between the type of the compiled expression and the expected result type.

Fixes #1915

[PathogenDavid]

Just one minor thing, good either way.