Added Activity.EnumerateEvents extension

[CodeBlanch]

Added an extension method for enumerating Activity Events without an allocation. Updated Jaeger & Zipkin to use it. A bit of renaming inside Jaeger & Zipkin to make their usage of enumeration extensions consistent.

Method	Mean	Error	StdDev	Gen 0	Gen 1	Gen 2	Allocated
EnumerateNonemptyActivityEvents	79.39 ns	1.429 ns	2.427 ns	0.0067	-	-	56 B
EnumerateEventsNonemptyActivityEvents	46.03 ns	0.502 ns	0.445 ns	-	-	-	-

TODOs:

• CHANGELOG.md updated for non-trivial changes
• Changes in public API reviewed

[codecov]

Codecov Report

Merging #1319 into master will increase coverage by 0.02%.
The diff coverage is 94.33%.

@@            Coverage Diff             @@
##           master    #1319      +/-   ##
==========================================
+ Coverage   78.63%   78.65%   +0.02%     
==========================================
  Files         219      219              
  Lines        6253     6265      +12     
==========================================
+ Hits         4917     4928      +11     
- Misses       1336     1337       +1     

Impacted Files	Coverage Δ
...plementation/ZipkinActivityConversionExtensions.cs	92.23% <85.71%> (+0.15%)	⬆️
src/OpenTelemetry.Api/Trace/ActivityExtensions.cs	98.85% <100.00%> (+0.25%)	⬆️
....Jaeger/Implementation/JaegerActivityExtensions.cs	96.71% <100.00%> (+0.50%)	⬆️
src/OpenTelemetry/Internal/EnumerationHelper.cs	88.57% <0.00%> (-2.86%)	⬇️