Make ZipkinExporter and JaegerExporter internal

[alanwest]

In #1528 I made the OtlpExporter internal. This does the same for the Zipkin and Jaeger exporters.

The thought is that adding and configuring these exporters is to be done through their respective TracerProviderBuilder extension methods. I also believe there is a plan to apply the pattern started with Zipkin in #1504 to both Jaeger and Otlp for configuring Simple vs. Batch export.

[codecov]

Codecov Report

Merging #1540 (b1187b5) into master (c409b89) will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1540      +/-   ##
==========================================
- Coverage   81.01%   80.97%   -0.04%     
==========================================
  Files         235      235              
  Lines        6414     6414              
==========================================
- Hits         5196     5194       -2     
- Misses       1218     1220       +2     

Impacted Files	Coverage Δ
...rc/OpenTelemetry.Exporter.Jaeger/JaegerExporter.cs	84.94% <ø> (ø)
...rc/OpenTelemetry.Exporter.Zipkin/ZipkinExporter.cs	88.04% <ø> (ø)
...emetry.Api/Internal/OpenTelemetryApiEventSource.cs	76.47% <0.00%> (-5.89%)	⬇️
...ter.ZPages/Implementation/ZPagesActivityTracker.cs	97.14% <0.00%> (-2.86%)	⬇️
...us/Implementation/PrometheusExporterEventSource.cs	72.72% <0.00%> (+9.09%)	⬆️

[cijothomas]

I also believe there is a plan to apply the pattern started with Zipkin in #1504 to both Jaeger and Otlp for configuring Simple vs. Batch export.

Yes. @Austin-Tan is adding this.