Add SpanExporter::ForceFlush and default implementation

[ymtdzzz]

Add SpanExporter::ForceFlush, and default implementation not to affect existing Exporters.

Fixes #836

[codecov]

Codecov Report

Merging #845 (7d5ad92) into main (b3e6e6a) will increase coverage by 0.1%.
The diff coverage is 80.0%.

@@           Coverage Diff           @@
##            main    #845     +/-   ##
=======================================
+ Coverage   68.7%   68.8%   +0.1%     
=======================================
  Files        115     115             
  Lines       9346    9371     +25     
=======================================
+ Hits        6427    6456     +29     
+ Misses      2919    2915      -4     

Impacted Files	Coverage Δ
opentelemetry-api/src/trace/context.rs	49.1% <ø> (ø)
opentelemetry-jaeger/src/exporter/collector.rs	20.0% <0.0%> (+0.1%)	⬆️
opentelemetry-sdk/src/export/trace/mod.rs	82.3% <0.0%> (-17.7%)	⬇️
opentelemetry-sdk/src/trace/tracer.rs	90.6% <0.0%> (ø)
opentelemetry-sdk/src/trace/sampler.rs	95.8% <100.0%> (+2.4%)	⬆️
opentelemetry-api/src/trace/tracer.rs	66.6% <0.0%> (+5.3%)	⬆️

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[jtescher]

Looks good, thanks @ymtdzzz