[Trace SDK] Implement builders (open-telemetry#1393)

Fixed bazel build, clang-format.

Format cleanup.

Use std::unique_ptr instead of nostd::unique_ptr

Continued, implemented:
- JaegerExporterFactory,
- InMemorySpanExporterFactory,
- RandomIdGeneratorFactory.

Added doc.
Makefile format.

Misc cleanup.

Implemented code review comments:
- reworded docs/cpp-sdk-factory-design.md,
- re organized includes,
- added TracerContextFactory.

Also:
- renamed Build() methods to Create(),
  for consistency with HttpClientFactory::Create(),
- added doxygen comments,
- added CHANGELOG entry.

Conflicts:
	CHANGELOG.md
	exporters/otlp/include/opentelemetry/exporters/otlp/otlp_http_exporter.h

Implement code review comments:
- Factory methods should return a unique_ptr.
- Used TracerContextFactory in examples.

Implement code review comments:
- avoid copy of resource parameters (used const reference)
- added unit tests to verify build sanity
- moved JsonBytesMappingKind and HttpRequestContentType
  to their own header, to avoid the dependency
  on protobuf from otlp_http_client.h

Fixed makefiles.

Unit test cleanup

CMake cleanup.

Reverted the renaming of library opentelemetry_exporter_in_memory to
opentelemetry_exporter_memory_trace.

The name opentelemetry_exporter_in_memory is used externally,
in cmake/opentelemetry-cpp-config.cmake.in

Format makefile.

Fixed grammar in documentation.