Support Custom span-id and trace-id generator

[lalitb]

Fixes #677

Changes

• Add abstract interface for a span-id and trace-id generation.
• Convert existing logic for random span-id and trace-id as the default implementation for the above interface.
• Modify TracerProvider and TracerContext constructor to support injecting the custom id generator.

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 #681 (5c9c35e) into main (ae060d1) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #681      +/-   ##
==========================================
+ Coverage   94.64%   94.67%   +0.02%     
==========================================
  Files         206      208       +2     
  Lines        9382     9407      +25     
==========================================
+ Hits         8880     8906      +26     
+ Misses        502      501       -1     

Impacted Files	Coverage Δ
sdk/include/opentelemetry/sdk/trace/id_generator.h	100.00% <100.00%> (ø)
sdk/include/opentelemetry/sdk/trace/tracer.h	100.00% <100.00%> (ø)
sdk/src/trace/random_id_generator.cc	100.00% <100.00%> (ø)
sdk/src/trace/span.cc	88.88% <100.00%> (-0.70%)	⬇️
sdk/src/trace/tracer_context.cc	80.00% <100.00%> (+5.00%)	⬆️
sdk/src/trace/tracer_provider.cc	78.94% <100.00%> (+2.47%)	⬆️
sdk/test/trace/tracer_provider_test.cc	100.00% <100.00%> (ø)
sdk/test/trace/tracer_test.cc	100.00% <100.00%> (ø)
... and 1 more

[eyjohn]

Overall looks great to land, some minor nitpicks,

Minor question about constness

[eyjohn]

nit: Google Mock would be great to use here, I noticed we don't have it in the repo but EXPECT_CALL and return values can all be done in the test body which would be nice