feat: check validity of trace- and span-id in context update from header

[JoshuaMoelans]

Analysis of #1012 showed a potential way for users to send invalid trace and span id values. This can happen when a customer uses sentry_transaction_context_update_from_header(...) which reads a string that looks like traceid-spanid(-sampled)?. The only formatting that currently gets checked is whether there is a first -.

Although these are never used in an unsafe context within our code, it still makes sense to reject the invalid formats on our end rather than relying on Relay to reject them.

The formats we expect are the following (from our docs):

• trace_id = 32 hex characters
• span_id = 16 hex characters

ToDo

• Make sure we agree on this approach
• Add a warning mechanism when an invalid input is received
• Update the relevant docs page

[github-actions]

	Messages
📖	Do not forget to update Sentry-docs with your feature once the pull request gets approved.

Generated by 🚫 dangerJS against 271b06f

[codecov]

Codecov Report

Attention: Patch coverage is 96.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 81.78%. Comparing base (dbb9580) to head (271b06f).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1046      +/-   ##
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- Coverage   83.33%   81.78%   -1.56%     
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  Files          53       53              
  Lines        6338     6363      +25     
  Branches     1198     1207       +9     
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- Hits         5282     5204      -78     
- Misses       1042     1046       +4     
- Partials       14      113      +99     

[JoshuaMoelans]

ToDo: update the docs (think it is this page) with a valid header.

[supervacuus]

I went ahead with the merge and created a docs issue as a follow-up to your todo item: getsentry/sentry-docs#11504