Prototype of PipelineOptions

[tom-andersen]

Currently this is a prototype of pipeline and stage options.

This is open for discussion and adjustment to pattern. With some refinement, this can become to part of the preview.

• Change FindNearestOptions to wither style.
• Fix encoding of Unnest stage to proto, such that args and options match specifrication.
• Add internal options object to superclass of stages, and use it to standardize encoding.
• Reduce logic in individual stages, by having them only construct the proto stage args, instead of entire stage proto.
• Use final and static where possible to encapsulate, control extension, and generally improve performance.
• Add internal encodeValue overloads to simplify code path.
• Add base class AbstractOptions as parent to all options. This will provide escape hatch methods, and subclasses can contain convenience methods for known options.
• Refactor FindNearest DistanceMeasure to use simpler constants. This reduces numbers of classes, code, and avoid continually creating new objects at runtime.
• Implement some known "hints". This is incomplete, and needs to be expanded upon as backend defines this more clearly.