Make SpanRef public

[ramosbugs]

#505 improved local span performance by moving the mutex into the
Context (via SynchronizedSpan) and introduced a SpanRef type
returned by TraceContextExt::span(). However, SpanRef is not currently
exported via the public API, which precludes passing it around easily within
client code. This PR exports that type alongside TraceContextExt.

[linux-foundation-easycla]

The committers are authorized under a signed CLA.

• ✅ David Ramos (47079b2)

[TommyCpp]

Good catch! Thanks

[codecov]

Codecov Report

Merging #600 (47079b2) into main (55d798e) will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##            main    #600   +/-   ##
=====================================
  Coverage   55.0%   55.0%           
=====================================
  Files        100     100           
  Lines       8890    8890           
=====================================
  Hits        4895    4895           
  Misses      3995    3995           

Impacted Files	Coverage Δ
opentelemetry/src/trace/mod.rs	71.1% <ø> (ø)

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