Rename AddActivitySource to AddSource, use params

[reyang]

Follow up on:

• Refactor TracerProvider to make configuration easy. #1027 (comment)
• Refactor TracerProvider to make configuration easy. #1027 (comment)
• Optimize the flow #1032 (comment)

Changes

• Renamed AddActivitySource to AddSource
• Take the params approach.

I will send a separate PR to clean up the CHANGELOG.md as it is becoming messy now.

Please provide a brief description of the changes here. Update the
CHANGELOG.md for non-trivial changes.

For significant contributions please make sure you have completed the following items:

• Design discussion issue #
• Changes in public API reviewed

[codecov]

Codecov Report

Merging #1036 into master will decrease coverage by 0.05%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master    #1036      +/-   ##
==========================================
- Coverage   77.17%   77.11%   -0.06%     
==========================================
  Files         221      221              
  Lines        6164     6162       -2     
==========================================
- Hits         4757     4752       -5     
- Misses       1407     1410       +3     

Impacted Files	Coverage Δ
...umentation.Http/TracerProviderBuilderExtensions.cs	100.00% <ø> (ø)
src/OpenTelemetry/Trace/TracerProviderBuilder.cs	89.18% <66.66%> (+9.70%)	⬆️
...ation.SqlClient/TracerProviderBuilderExtensions.cs	100.00% <100.00%> (ø)
...ckExchangeRedis/TracerProviderBuilderExtensions.cs	100.00% <100.00%> (ø)
src/OpenTelemetry/Metrics/CounterMetricSdkBase.cs	83.33% <0.00%> (-13.34%)	⬇️
...nTelemetry/Internal/OpenTelemetrySdkEventSource.cs	82.35% <0.00%> (-4.42%)	⬇️