Store current Span instead of local and remote SpanContext in context.Context

[MrAlias]

Now that the SpanContext has a remote identifier, storing a Span's SpanContext two separate ways in a Context (one for local another for remote) is unnecessary and adds complication throughout the project when determining heredity of a Span. This moves to storing the Span directly in the Context uniformly (for both local and remote) as current Span. In the process, it updates the getter/setter functionality in the trace package provides and removes the distributed heredity logic throughout the project with just using the current Span as the parent if it exists.

Resolves #1182

[codecov]

Codecov Report

Merging #1731 (03c3bbe) into main (c61f4b6) will increase coverage by 0.0%.
The diff coverage is 93.3%.

@@          Coverage Diff          @@
##            main   #1731   +/-   ##
=====================================
  Coverage   77.9%   78.0%           
=====================================
  Files        131     132    +1     
  Lines       6953    6941   -12     
=====================================
- Hits        5422    5417    -5     
+ Misses      1281    1274    -7     
  Partials     250     250           

Impacted Files	Coverage Δ
bridge/opentracing/bridge.go	47.8% <0.0%> (ø)
trace/noop.go	57.1% <ø> (ø)
trace/trace.go	86.4% <ø> (+0.2%)	⬆️
bridge/opentracing/internal/mock.go	73.6% <100.0%> (ø)
oteltest/config.go	92.1% <100.0%> (-0.4%)	⬇️
oteltest/tracer.go	89.6% <100.0%> (-1.0%)	⬇️
sdk/trace/span.go	90.4% <100.0%> (ø)
trace/context.go	100.0% <100.0%> (ø)
trace/nonrecording.go	100.0% <100.0%> (ø)
... and 2 more