Restructure Ext folder

[jajanet]

This PR is to restructure and move all HTTP server and zPages code so that they're grouped in separate locations, meaning they can be used and included by themselves as needed. They're currently tightly coupled, so adding the HTTP server headers also includes zPages--this is not ideal. The zPages README location was also odd since there were 3 zpages folders (in include, src, and test), and now the new location (1 single zpages folder) makes more sense.

With that, this also ensures future ext additions aren't tightly coupled with the HTTP server and zPages. The number of CMake files are reduced, making future maintenance and development easier.

#288 should be merged before this.

[codecov]

Codecov Report

Merging #292 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #292   +/-   ##
=======================================
  Coverage   93.93%   93.93%           
=======================================
  Files         146      146           
  Lines        6564     6564           
=======================================
  Hits         6166     6166           
  Misses        398      398           

Impacted Files	Coverage Δ
...lude/opentelemetry/ext/zpages/latency_boundaries.h	100.00% <ø> (ø)
...de/opentelemetry/ext/zpages/threadsafe_span_data.h	96.87% <ø> (ø)
...ges/include/opentelemetry/ext/zpages/tracez_data.h	100.00% <ø> (ø)
...nclude/opentelemetry/ext/zpages/tracez_processor.h	100.00% <ø> (ø)
ext/zpages/src/tracez_data_aggregator.cc	100.00% <ø> (ø)
ext/zpages/src/tracez_processor.cc	95.23% <ø> (ø)
ext/zpages/test/threadsafe_span_data_test.cc	100.00% <ø> (ø)
ext/zpages/test/tracez_data_aggregator_test.cc	97.34% <ø> (ø)
ext/zpages/test/tracez_processor_test.cc	98.70% <ø> (ø)

[jajanet]

Likely replacing the HTTP client in the future, so restructuring changes no longer needed. See #309