feat(tracing): Add Span.SetDynamicSamplingContext method

[tonyo]

This PR adds Span.SetDynamicSamplingContext method that allows to set the given dynamic sampling context on a transaction. For spans it does nothings.

This is needed for the OTel work (#537).

[codecov]

Codecov Report

Base: 84.19% // Head: 83.97% // Decreases project coverage by -0.23% ⚠️

Coverage data is based on head (0770aff) compared to base (4fad7b7).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #539      +/-   ##
==========================================
- Coverage   84.19%   83.97%   -0.23%     
==========================================
  Files          23       23              
  Lines        2816     2820       +4     
==========================================
- Hits         2371     2368       -3     
- Misses        365      370       +5     
- Partials       80       82       +2     

Impacted Files	Coverage Δ
tracing.go	82.43% <100.00%> (+0.17%)	⬆️
integrations.go	89.01% <0.00%> (-2.32%)	⬇️
transport.go	78.37% <0.00%> (-0.85%)	⬇️

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