Add basic cancellation support for async implementations

[Nevay]

Allows async implementations to subscribe to timeouts etc.

[codecov]

Codecov Report

Merging #782 (b69dc19) into main (2f6658f) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               main     #782   +/-   ##
=========================================
  Coverage     79.52%   79.52%           
  Complexity     1405     1405           
=========================================
  Files           160      160           
  Lines          3468     3468           
=========================================
  Hits           2758     2758           
  Misses          710      710           

Flag	Coverage Δ
7.4	79.52% <100.00%> (ø)
8.0	79.59% <100.00%> (ø)
8.1	79.59% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
.../SDK/Trace/Behavior/SpanExporterDecoratorTrait.php	100.00% <100.00%> (ø)
src/SDK/Trace/Behavior/SpanExporterTrait.php	100.00% <100.00%> (ø)
src/SDK/Trace/SpanProcessor/BatchSpanProcessor.php	100.00% <100.00%> (ø)
src/SDK/Trace/SpanProcessor/MultiSpanProcessor.php	100.00% <100.00%> (ø)
src/SDK/Trace/SpanProcessor/NoopSpanProcessor.php	100.00% <100.00%> (ø)
...rc/SDK/Trace/SpanProcessor/SimpleSpanProcessor.php	100.00% <100.00%> (ø)
src/SDK/Trace/TracerProvider.php	100.00% <100.00%> (ø)
src/SDK/Trace/TracerSharedState.php	100.00% <100.00%> (ø)

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[brettmc]

@Nevay looks good to me, there's a conflict to be resolved.

[Nevay]

Rebased and removed cancellation from SpanExporterTrait::doExport(); the trait isn't useful for async implementations as ::forceFlush() and ::shutdown() should block.

[ForceFlush] is a hint to ensure that the export [...] SHOULD be completed as soon as possible, preferably before returning from this method.