Add support for concurrent exports

[Nevay]

Related to #788.

The return of Export() is implementation specific. In what is idiomatic for the language the Exporter must send an ExportResult to the Processor. ExportResult has values of either Success or Failure

[codecov]

Codecov Report

Merging #790 (903d9bc) into main (e946dc7) will decrease coverage by 0.09%.
The diff coverage is 63.63%.

@@             Coverage Diff              @@
##               main     #790      +/-   ##
============================================
- Coverage     79.52%   79.43%   -0.10%     
- Complexity     1405     1407       +2     
============================================
  Files           160      161       +1     
  Lines          3468     3472       +4     
============================================
  Hits           2758     2758              
- Misses          710      714       +4     

Flag	Coverage Δ
7.4	79.43% <63.63%> (-0.10%)	⬇️
8.0	79.50% <63.63%> (-0.10%)	⬇️
8.1	79.50% <63.63%> (-0.10%)	⬇️

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Impacted Files	Coverage Δ
src/SDK/Common/Future/CompletedFuture.php	0.00% <0.00%> (ø)
.../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)
...rc/SDK/Trace/SpanProcessor/SimpleSpanProcessor.php	100.00% <100.00%> (ø)

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[brettmc]

Looks good to me. Futures look quite similar to Promises, I was going to ask why not use promises (esp given recent cancellation work), but I think I talked myself out of it.
I'll wait a day or two to see if anybody else wants to also review.