Support determining parent span from Context while creating new Span

[lalitb]

Fixes #955

Changes

This adds support for passing Context wrapping parent Span object while creating new Span.

• In order to not avoid breaking the existing customers code, the legacy api supporting passing SpanContext object is also kept in place. Also to avoid storing two different variables for Context and SpanContext inside StartSpanOptions struct, variant object is introduced within this struct.
• Slight restructuring in header files was required to avoid cyclic dependency in header files ( i.e span.h requiring context/context.h and context/context.h requiring span.h through context/context_value.h) : this required adding span_startoptions.h and span_metadata.h.

cc @jsuereth

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

Merging #969 (1bf59ef) into main (703576c) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #969      +/-   ##
==========================================
+ Coverage   95.36%   95.38%   +0.03%     
==========================================
  Files         161      161              
  Lines        6780     6817      +37     
==========================================
+ Hits         6465     6502      +37     
  Misses        315      315              

Impacted Files	Coverage Δ
api/include/opentelemetry/trace/span.h	100.00% <ø> (ø)
api/include/opentelemetry/trace/tracer.h	100.00% <ø> (ø)
sdk/src/trace/tracer.cc	90.25% <100.00%> (+2.75%)	⬆️
sdk/test/trace/tracer_test.cc	99.52% <100.00%> (+0.04%)	⬆️

[lalitb]

build failed due to merge conflict with #966 , have resolved them now.