Mark Activity extension methods not required by spec as internal

[cijothomas]

Fixes #1544 .

Need to do a follow up PR to possible split Benchmarks as it has ambigous ref to same extension method. Temporarily removed the file, to be added back next PR.

Changes

Please provide a brief description of the changes here.

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

• CHANGELOG.md updated for non-trivial changes
• Design discussion issue #
• Changes in public API reviewed

[codecov]

Codecov Report

Merging #1545 (dfc35a7) into master (4c7d2ae) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1545      +/-   ##
==========================================
+ Coverage   81.01%   81.04%   +0.03%     
==========================================
  Files         235      236       +1     
  Lines        6414     6419       +5     
==========================================
+ Hits         5196     5202       +6     
+ Misses       1218     1217       -1     

Impacted Files	Coverage Δ
...Telemetry.Api/Internal/ActivityHelperExtensions.cs	100.00% <100.00%> (ø)
src/OpenTelemetry.Api/Trace/ActivityExtensions.cs	93.75% <100.00%> (-5.35%)	⬇️
...us/Implementation/PrometheusExporterEventSource.cs	72.72% <0.00%> (+9.09%)	⬆️

[cijothomas]

@CodeBlanch @reyang Please review. This will have cause a lot of merge conflicts with other PRs if left for a long time :D

[CodeBlanch]

LGTM