feat(tracing): Add a method to get transaction for given span: GetTransaction()

[tonyo]

Add a function to get the transaction span for the given span: Span.GetTransaction()
This is needed for a proper fix for #553.

Things left to agree on: Function name
In Python, for example, there's a containing_transaction method on Spans and Transactions.

[codecov]

Codecov Report

Base: 75.47% // Head: 75.48% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (76ebf1b) compared to base (d362c36).
Patch coverage: 71.42% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #558      +/-   ##
==========================================
+ Coverage   75.47%   75.48%   +0.01%     
==========================================
  Files          37       37              
  Lines        3751     3765      +14     
==========================================
+ Hits         2831     2842      +11     
  Misses        808      808              
- Partials      112      115       +3     

Impacted Files	Coverage Δ
tracing.go	87.47% <71.42%> (-0.52%)	⬇️
transport.go	78.37% <0.00%> (-0.85%)	⬇️
span_recorder.go	89.65% <0.00%> (+13.79%)	⬆️

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[cleptric]

I think this is mostly OK for the time being.
No hard feelings about the name, it's Span::getTransaction in PHP as well.

Slightly OT, but I think this is a good example of why we should add proper abstractions for Transactions/Spans.
https://github.com/getsentry/sentry-php/blob/bfa3193944fe385af7d6b6b063962fc45956a817/src/Tracing/Span.php#L390-L406

I kinda like the way all parent properties are explicitly set on a child span.