Sampling: Introduce decision/flag to not propagate a span.

An idea that I had for https://github.com/open-telemetry/opentelemetry-specification/pull/1653#issuecomment-830025528: With the current sampling decisions, you cannot suppress propagation of a Span. You can only influence the sampled flag on the traceflags. In some situations it might be desired to not propagate the span at all cross-process, i.e. cause inject(context) to be a no-op (for the Span part of the context).

This could be added as a new sampling decision that is otherwise equivalent to DROP, or it could be added as an additional boolean flag orthogonal to the current sampling decisions to also allow e.g. RECORDING spans that are only propagated in-process (could be used in combination with the URL which might be available to samplers to not propagate span contexts to external services).

Start with the sampling decisions and context propagation behavior described in the issue, then read the referenced OpenTelemetry specification PR and comment. Compare the proposed new decision with the orthogonal-flag option, and define the resulting inject(context) behavior and compatibility expectations as the completion criteria.