refactor: context only parenting

[mwear]

This PR is a draft showing what changes would be required to restrict explicit span parenting to context only (rather than allowing a span, or context). This is an effort towards validating that: open-telemetry/opentelemetry-specification#510 is viable in Ruby.

There were two options to provide an explicit parent to start_span and friends. One was named with_parent and expected an argument to be a span. The other was named with_parent_context where the argument was a context. In this PR we removed with_parent_context and updated with_parent to accept a context rather than a span.

I also added a Tracer#context_with_span convenience method which is the equivalent of ctx = tracer.with_span(span) { |_, c| c }, which was something I found myself doing in tests when I removed explicit span based parenting.

The original with_parent option was only used in tests. Instrumentation used with_parent_context, further validating context based parenting want we want.

[fbogsany]

I realize this is just a prototype, but it looks like a good improvement to me 👍

[fbogsany]

open-telemetry/opentelemetry-specification#875 also requires on_start for span processors to accept a parent_context parameter. This requires a little refactoring because Span#initialize only has a SpanContext and not the parent Context.

[mwear]

This is ready for review and can merge when folks are comfortable with it. The corresponding spec changes merged earlier today: open-telemetry/opentelemetry-specification#875.

[robertlaurin]

This makes sense to me.