feat: configure TLS with environment variables.

[jvanz]

Updates the opentelemetry-otlp crate to allow users to configure TLS using environment variables. Removing the need to crating the TLS config object and defining it with the with_tls_config method. In the same way other OTLP libraries does (e.g. go lang).

Partially fixes #774

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

Attention: Patch coverage is 2.79720% with 139 lines in your changes missing coverage. Please review.

Project coverage is 76.8%. Comparing base (6e1032f) to head (887a6a6).

Files with missing lines	Patch %	Lines
opentelemetry-otlp/src/exporter/tonic/mod.rs	0.0%	122 Missing ⚠️
opentelemetry-otlp/src/exporter/mod.rs	19.0%	17 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #2465     +/-   ##
=======================================
- Coverage   77.1%   76.8%   -0.4%     
=======================================
  Files        124     124             
  Lines      23021   23124    +103     
=======================================
+ Hits       17771   17775      +4     
- Misses      5250    5349     +99     

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