Allow Custom Samplers

[hdost]

This adds the ability for users to combine custom samplers with the built-in Parent-Based Sampler.

[codecov]

Codecov Report

Merging #833 (3b20f08) into main (b3e6e6a) will increase coverage by 0.1%.
The diff coverage is 96.0%.

@@           Coverage Diff           @@
##            main    #833     +/-   ##
=======================================
+ Coverage   68.7%   68.9%   +0.1%     
=======================================
  Files        115     115             
  Lines       9346    9369     +23     
=======================================
+ Hits        6427    6461     +34     
+ Misses      2919    2908     -11     

Impacted Files	Coverage Δ
opentelemetry-sdk/src/trace/tracer.rs	90.6% <0.0%> (ø)
opentelemetry-sdk/src/trace/sampler.rs	95.8% <100.0%> (+2.4%)	⬆️
opentelemetry-jaeger/src/exporter/mod.rs	53.9% <0.0%> (+2.6%)	⬆️
opentelemetry-api/src/trace/tracer.rs	66.6% <0.0%> (+5.3%)	⬆️

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