Add zPages Tracez Processor

[jajanet]

This PR is for the TraceZ span processor, which will interface with a TracerProvider/Tracer on the user end to send spans to the TraceZ aggregator and later TraceZ HTTP server. This will help us eventually provide an easy way for users to see tracing information for debugging application issues (like deadlocks and high latency) without needing third party tracing systems. See the README and design doc

Files include:

1. Processor code with added methods relevant to TraceZ for zPages. This includes storing and providing getters for running/newly completed spans (completed spans that haven't been sent before). Received spans/recordables are casted as span_data. The aggregator will regularly call the processor to minimize potential issues with memory from storing spans.
2. Comprehensive tests for the functionality described above in different scenarios, including when completed spans may want to be stored cumulatively or non-cumulatively and different timing of spans.
3. Bazel and CMake build files

Future PRs for TraceZ will be for its data aggregator and HTTP server

[codecov]

Codecov Report

Merging #164 into master will increase coverage by 0.57%.
The diff coverage is 99.36%.

@@            Coverage Diff             @@
##           master     #164      +/-   ##
==========================================
+ Coverage   94.88%   95.45%   +0.57%     
==========================================
  Files          84       87       +3     
  Lines        2171     2488     +317     
==========================================
+ Hits         2060     2375     +315     
- Misses        111      113       +2     

Impacted Files	Coverage Δ
ext/test/zpages/tracez_processor_test.cc	99.30% <99.30%> (ø)
...nclude/opentelemetry/ext/zpages/tracez_processor.h	100.00% <100.00%> (ø)
ext/src/zpages/tracez_processor.cc	100.00% <100.00%> (ø)

[reyang]

Need to clarify the concurrency consideration.

[reyang]

LGTM assuming comments will be resolved.

[reyang]

@jajanet would you rebase please? Thanks.