fix: Pass SentryTracer to span child.

[brustolin]

📜 Description

Every new span child created need a reference to SentryTracer to be able to also create child.

💡 Motivation and Context

To fix a problem.

💚 How did you test it?

Unit test.

📝 Checklist

• I reviewed the submitted code
• I added tests to verify the changes
• I updated the CHANGELOG if needed
• I updated the docs if needed
• Review from the native team if needed
• No breaking changes

[codecov-io]

Codecov Report

Merging #1040 (ebffbaa) into master (ca91364) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1040   +/-   ##
=======================================
  Coverage   94.88%   94.88%           
=======================================
  Files          93       93           
  Lines        4083     4083           
=======================================
  Hits         3874     3874           
  Misses        209      209           

Impacted Files	Coverage Δ
Sources/Sentry/SentryTracer.m	92.85% <100.00%> (ø)

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[marandaneto]

@brustolin please fill-up the Checklist accordingly

[marandaneto]

LGTM