Move the Event type from the API to the SDK

[MrAlias]

This type is not used in the API. It is used in the SDK and then through the processing pipelines. Move it to the package that originates its use.

Mostly resolves, but there are a few remaining questions for #1845
Part of #1380

[codecov]

Codecov Report

Merging #1846 (922b304) into main (e399d35) will increase coverage by 0.0%.
The diff coverage is 80.0%.

@@          Coverage Diff          @@
##            main   #1846   +/-   ##
=====================================
  Coverage   78.6%   78.6%           
=====================================
  Files        137     137           
  Lines       7305    7305           
=====================================
+ Hits        5746    5749    +3     
+ Misses      1315    1313    -2     
+ Partials     244     243    -1     

Impacted Files	Coverage Δ
exporters/otlp/internal/transform/span.go	98.1% <ø> (ø)
exporters/trace/zipkin/model.go	99.1% <ø> (ø)
trace/trace.go	86.7% <ø> (ø)
sdk/trace/span.go	94.0% <75.0%> (ø)
exporters/otlp/internal/otlptest/data.go	88.0% <100.0%> (ø)
exporters/trace/jaeger/jaeger.go	92.8% <0.0%> (+0.5%)	⬆️
sdk/trace/batch_span_processor.go	87.1% <0.0%> (+1.5%)	⬆️