Move integration settings into tracing namespace

[delner]

For consistency, this pull request moves the Contrib extensions for settings from Datadog.configuration to Datadog.configuration.tracing. This was done to ensure usage like Datadog.configuration.tracing[:rails], which is unambiguous as to which feature it belongs.

All integrations have been updated accordingly, and a change the the UpgradeGuide has been added to match.

[codecov-commenter]

Codecov Report

Merging #1922 (c7f5ae6) into master (32fc71b) will decrease coverage by 0.00%.
The diff coverage is 92.50%.

@@            Coverage Diff             @@
##           master    #1922      +/-   ##
==========================================
- Coverage   97.51%   97.50%   -0.01%     
==========================================
  Files         997      997              
  Lines       48737    48747      +10     
==========================================
+ Hits        47524    47532       +8     
- Misses       1213     1215       +2     

Impacted Files	Coverage Δ
lib/datadog/appsec/contrib/rails/patcher.rb	44.06% <0.00%> (ø)
lib/datadog/core/configuration/settings.rb	100.00% <ø> (ø)
...racing/contrib/active_model_serializers/patcher.rb	94.11% <0.00%> (ø)
lib/datadog/tracing/contrib/aws/patcher.rb	96.15% <0.00%> (ø)
...current_ruby/context_composite_executor_service.rb	95.65% <0.00%> (ø)
lib/datadog/tracing/contrib/integration.rb	100.00% <ø> (ø)
lib/datadog/tracing/contrib/qless/patcher.rb	95.00% <0.00%> (ø)
lib/datadog/tracing/contrib/qless/qless_job.rb	79.48% <0.00%> (ø)
lib/datadog/tracing/contrib/rake/patcher.rb	94.44% <0.00%> (ø)
lib/datadog/tracing/contrib/sequel/database.rb	97.05% <ø> (ø)
... and 85 more

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