fix(tracing): Add WithTransactionSource() span option, deprecate TransctionSource()

[tonyo]

This deprecates TransctionSource() (which has a typo and therefore might confuse people) in favor of WithTransactionSource(). TransactionSource type already exists, hence the "With" prefix.

TransctionSource() is not removed right away, instead we should add a deprecation note in the changelog of the next release.

NOTE:

1. Open to alternative suggestions on the name of this span option
2. For consistency, we might want to prefix all applicable span options with "With". Also, open to discussion.

[cleptric]

As a follow-up, let's rename all SpanOptions functions to With*.

[codecov]

Codecov Report

Patch coverage: 66.66% and project coverage change: +0.03 🎉

Comparison is base (9f9f913) 79.91% compared to head (2849285) 79.94%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #611      +/-   ##
==========================================
+ Coverage   79.91%   79.94%   +0.03%     
==========================================
  Files          38       38              
  Lines        3873     3875       +2     
==========================================
+ Hits         3095     3098       +3     
  Misses        671      671              
+ Partials      107      106       -1     

Impacted Files	Coverage Δ
tracing.go	88.70% <50.00%> (-0.38%)	⬇️
http/sentryhttp.go	87.27% <100.00%> (ø)

... and 1 file with indirect coverage changes

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