Renaming ITextFormat to IPropagator

[eddynaka]

Fixes #.

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 #1190 into master will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1190      +/-   ##
==========================================
- Coverage   77.88%   77.87%   -0.02%     
==========================================
  Files         219      219              
  Lines        6304     6304              
==========================================
- Hits         4910     4909       -1     
- Misses       1394     1395       +1     

Impacted Files	Coverage Δ
...nTelemetry.Api/Context/Propagation/B3Propagator.cs	85.71% <ø> (ø)
...metry.Api/Context/Propagation/BaggagePropagator.cs	83.92% <ø> (ø)
...metry.Api/Context/Propagation/TextMapPropagator.cs	76.23% <ø> (ø)
...try.Api/Context/Propagation/CompositePropagator.cs	81.81% <100.00%> (ø)
...rumentation.AspNet/AspNetInstrumentationOptions.cs	100.00% <100.00%> (ø)
...umentation.AspNet/Implementation/HttpInListener.cs	94.25% <100.00%> (ø)
...ion.AspNetCore/AspNetCoreInstrumentationOptions.cs	100.00% <100.00%> (ø)
...tation.AspNetCore/Implementation/HttpInListener.cs	90.52% <100.00%> (ø)
...mentation.Http/HttpClientInstrumentationOptions.cs	100.00% <100.00%> (ø)
...tp/Implementation/HttpHandlerDiagnosticListener.cs	87.83% <100.00%> (ø)
... and 2 more