API: Pipeline.of* methods (and corresponding builder ctors) with direct class references instead of type hinting

[eledhwen]

Current implementation relies on type hinting for pipeline builder initialization:

• Pipeline.ofPayload relies on the provided Initializer signature, if none is provided it requires manual type hinting
• Pipeline.ofSimple requires manual type hinting

Type hinting is not intuitive (many devs don't even know it exists, and to their credit scenarios where they are relevant are few and far between) and is subject to type erasure.

We could probably upgrade this by requiring direct type references instead, which we could then eventually use for runtime type checks.