Remove distributed_tracing option namespacing

[marcotc]

2.0 Upgrade Guide notes

Programmatic configuration options have been made more consistent with their respective environment variables:

Previous	New	Environment Variable (unchanged)
tracing.distributed_tracing.propagation_extract_first	tracing.propagation_extract_first	DD_TRACE_PROPAGATION_EXTRACT_FIRST
tracing.distributed_tracing.propagation_extract_style	tracing.propagation_style_extract	DD_TRACE_PROPAGATION_STYLE_EXTRACT
tracing.distributed_tracing.propagation_inject_style	tracing.propagation_style_inject	DD_TRACE_PROPAGATION_STYLE_INJECT
tracing.distributed_tracing.propagation_style	tracing.propagation_style	DD_TRACE_PROPAGATION_STYLE

Motivation:
Environment variable naming is standard across all Datadog languages.
Our programmatic configuration API is an extra avenue for configuration for Ruby clients, and reflects the same options as the ones provided by environment variables.
The naming between the programmatic variant and the environment variable variant of the same option should be as close as possible.

Additional Notes:

How to test the change?

For Datadog employees:

• If this PR touches code that signs or publishes builds or packages, or handles
  credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

Unsure? Have a question? Request a review!

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d67d0a4) 98.11% compared to head (783aa86) 98.12%.
Report is 42 commits behind head on 2.0.

Additional details and impacted files

@@            Coverage Diff             @@
##              2.0    #3338      +/-   ##
==========================================
+ Coverage   98.11%   98.12%   +0.01%     
==========================================
  Files        1250     1243       -7     
  Lines       72401    72014     -387     
  Branches     3391     3376      -15     
==========================================
- Hits        71033    70662     -371     
+ Misses       1368     1352      -16     

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