Consider exposing Baggage Manager as functions

[carlosalberto]

As BaggageManager is not required in other SIGs API (as there won't be specialized implementations, as opposed to traces/metrics), consider removing this class and expose the functionality directly, e.g.

module Baggage
  def build(context:Context) # etc

This is also how these functions are exposed in Python, btw. Not a strong feeling though.