Add Support for Opencensus to OpenTelemetry migration

[zoercai]

Branched off @nilebox changes here: nilebox#1

Declare the ContextManager interface which allows for overriding the context storage in OpenCensus.
Default implementation keeps backward compatible behaviour using gRPC context with OpenCensus-specific key.

Added reflection for OpenTelemetry trace component which will be in the migration library.

[codecov-io]

Codecov Report

Merging #2059 into master will increase coverage by 0.03%.
The diff coverage is 95.12%.

@@             Coverage Diff              @@
##             master    #2059      +/-   ##
============================================
+ Coverage     83.05%   83.08%   +0.03%     
- Complexity     2249     2264      +15     
============================================
  Files           319      322       +3     
  Lines         10526    10557      +31     
  Branches       1048     1048              
============================================
+ Hits           8742     8771      +29     
- Misses         1446     1448       +2     
  Partials        338      338              

Impacted Files	Coverage Δ	Complexity Δ
.../java/io/opencensus/trace/unsafe/ContextUtils.java	100.00% <ø> (ø)	4.00 <0.00> (ø)
api/src/main/java/io/opencensus/trace/Tracing.java	95.23% <75.00%> (-4.77%)	7.00 <1.00> (ø)
...io/opencensus/trace/unsafe/ContextHandleUtils.java	90.90% <90.90%> (ø)	5.00 <5.00> (?)
...ain/java/io/opencensus/trace/CurrentSpanUtils.java	88.88% <100.00%> (+0.20%)	6.00 <1.00> (ø)
.../io/opencensus/trace/unsafe/ContextHandleImpl.java	100.00% <100.00%> (ø)	4.00 <4.00> (?)
...io/opencensus/trace/unsafe/ContextManagerImpl.java	100.00% <100.00%> (ø)	6.00 <6.00> (?)
...ontrib/logcorrelation/log4j2/ContextDataUtils.java	88.00% <100.00%> (ø)	8.00 <1.00> (ø)
...on/stackdriver/OpenCensusTraceLoggingEnhancer.java	86.36% <100.00%> (ø)	12.00 <1.00> (ø)
.../sleuth/v1x/OpenCensusSleuthSpanContextHolder.java	81.25% <100.00%> (+0.39%)	17.00 <0.00> (ø)
... and 2 more

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