Add xop proto uploader

[muir]

This is a first version of a protobuf uploader using the xop native protocol
It's not yet high performance but should be okay for now.
Test coverage is so-so.

[codecov-commenter]

Codecov Report

Base: 87.13% // Head: 87.19% // Increases project coverage by +0.05% 🎉

Coverage data is based on head (3f6e287) compared to base (e5a638c).
Patch coverage: 82.61% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #88      +/-   ##
==========================================
+ Coverage   87.13%   87.19%   +0.05%     
==========================================
  Files          45       48       +3     
  Lines        5124     5468     +344     
==========================================
+ Hits         4465     4768     +303     
- Misses        564      590      +26     
- Partials       95      110      +15     

Flag	Coverage Δ
go_tests	87.19% <82.61%> (+0.05%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
xopjson/models.go	68.42% <ø> (ø)
xoptest/xoptestutil/enums.go	80.00% <ø> (ø)
xoptrace/trace.go	83.80% <ø> (ø)
xoputil/skip.go	7.14% <0.00%> (ø)
xopat/attributes.go	51.16% <53.84%> (+4.86%)	⬆️
xopmiddle/utils.go	58.97% <66.66%> (ø)
xoptrace/hexbytes.go	74.14% <66.66%> (ø)
xopup/upload.go	75.29% <75.29%> (ø)
xopotel/otel.go	85.50% <80.00%> (ø)
span.go	84.74% <83.33%> (ø)
... and 19 more

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[miccagiann]

Awesome work!