Add SpanOptions::getParent

[jcchavezs]

Background

Most of the tracer implementations only support child_of reference type.

Problem

When implementing the startSpan method we need to iterate over all references in order to find the child_of reference.

Proposal

Having a SpanOptions::getParent encapsulating this method would be very useful.

Ping @tedsuo @felixfbecker @beberlei @lvht @yurishkuro

[yurishkuro]

can it be a private helper method instead? Making it part of SpanOptions extends the API surface.