Consider adding BuildObservable overload that returns typed sequence

[glopesdev]

Several aspects of building and running workflows from code remain cumbersome. One of them is getting the result of compiled observable sequences, which currently has to be done by calling Build to get the workflow expression tree, and then assembling and calling a lambda to finally generate the typed sequence.

BuildObservable provides a wrapper to take care of this boilerplate code, but it currently assumes the type of the result sequence is always Unit by inserting a UnitBuilder at the end, which disregards any actual output value from the workflow.

Both for testing and other applications where the type of the workflow is either known or expected, we could provide a generic overload allowing for explicit indication of the type of the observable sequence returned by the workflow. This overload would test if the output sequence is compatible with the provided type, and if so, build and compile a final observable with the specified type.