[Exporter.Stackdriver] Remove contrib from Stackdriver files

[utpilla]

Fixes #241

Changes

• Remove the word Contrib from the namespaces and test project

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

• Appropriate CHANGELOG.md updated for non-trivial changes

[codecov]

Codecov Report

Merging #513 (c07411d) into main (25f3c43) will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##            main    #513   +/-   ##
=====================================
  Coverage   0.00%   0.00%           
=====================================
  Files        167     167           
  Lines       5098    5098           
=====================================
  Misses      5098    5098           

Impacted Files	Coverage Δ
...r.Stackdriver/Implementation/ActivityExtensions.cs	0.00% <ø> (ø)
...y.Exporter.Stackdriver/Implementation/Constants.cs	0.00% <ø> (ø)
...r/Implementation/ExporterStackdriverEventSource.cs	0.00% <ø> (ø)
...kdriver/Implementation/GoogleCloudResourceUtils.cs	0.00% <ø> (ø)
...er/Implementation/StackdriverStatsConfiguration.cs	0.00% <ø> (ø)
...y.Exporter.Stackdriver/StackdriverTraceExporter.cs	0.00% <ø> (ø)
...ter.Stackdriver/TracerProviderBuilderExtensions.cs	0.00% <ø> (ø)
...elemetry.Exporter.Stackdriver/Utils/CommonUtils.cs	0.00% <ø> (ø)