[Span First #2]: Add envelope type to Dart layer

[buenaflor]

📜 Description

Span First initiative

💡 Motivation and Context

Add support for span envelope items in Dart's transport layer (for Android we need to change the native SDKs directly - iOS works already without modification)

Also the API fromSpansData is when we add the spans from the buffer similar to logs

💚 How did you test it?

Unit test and manual tests

📝 Checklist

• I reviewed submitted code
• I added tests to verify changes
• No new PII added or SDK only sends newly added PII if sendDefaultPii is enabled
• I updated the docs if needed
• All tests passing
• No breaking changes

🔮 Next steps

#skip-changelog

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (feat/span-first@de96551). Learn more about missing BASE report.

Additional details and impacted files

@@                Coverage Diff                 @@
##             feat/span-first    #3366   +/-   ##
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  Coverage                   ?   88.37%           
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  Files                      ?      290           
  Lines                      ?     9834           
  Branches                   ?        0           
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  Hits                       ?     8691           
  Misses                     ?     1143           
  Partials                   ?        0           

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[buenaflor]

not actually used yet but will be used by the span buffer (impl TBD)

[denrase]

LGTM