Include schema_url in FriendlySpanConverter

[sudol]

Regarding this issue: #1290

I believe this is the expected way to communicate the scheme used to a collector or downstream system attempting to analyze the telemetry, based on the documentation on the OpenTelemetry website.

Closes: #1290

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.48%. Comparing base (8a6dd47) to head (cc72bbf).

❗ Current head cc72bbf differs from pull request most recent head 51cb8b0. Consider uploading reports for the commit 51cb8b0 to get more accurate results

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1292      +/-   ##
============================================
+ Coverage     74.31%   74.48%   +0.17%     
+ Complexity     2391     2384       -7     
============================================
  Files           349      348       -1     
  Lines          7124     7099      -25     
============================================
- Hits           5294     5288       -6     
+ Misses         1830     1811      -19     

Flag	Coverage Δ
8.1	74.48% <100.00%> (+0.17%)	⬆️
8.2	74.48% <100.00%> (?)
8.3	74.48% <100.00%> (?)
8.4	?

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Files	Coverage Δ
...c/SDK/Trace/SpanExporter/FriendlySpanConverter.php	100.00% <100.00%> (ø)

... and 7 files with indirect coverage changes

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