Merge sdk/export/trace into sdk/trace

[MrAlias]

In order to eventually resolve an import cycle when returning a SpanSnapshot as a ReadOnlySpan in the SpanExporter, as is required by the specification, this merges the contents of the sdk/export/trace package into the sdk/trace package.

This does not move the sdk/export/metric module. This will likely need to be integrated in some way with the sdk/metric module, but given the experimental and unstable state of the metrics signal in OTel currently no action is taken yet.

Part of #1380

[codecov]

Codecov Report

Merging #1778 (9381d16) into main (c61e654) will not change coverage.
The diff coverage is 87.5%.

@@          Coverage Diff          @@
##            main   #1778   +/-   ##
=====================================
  Coverage   78.7%   78.7%           
=====================================
  Files        134     134           
  Lines       7116    7116           
=====================================
  Hits        5602    5602           
  Misses      1268    1268           
  Partials     246     246           

Impacted Files	Coverage Δ
exporters/otlp/internal/transform/span.go	98.1% <ø> (ø)
exporters/otlp/otlp.go	90.9% <ø> (ø)
exporters/otlp/otlpgrpc/driver.go	90.1% <ø> (ø)
exporters/otlp/otlphttp/driver.go	94.5% <ø> (ø)
exporters/otlp/protocoldriver.go	100.0% <ø> (ø)
exporters/stdout/exporter.go	17.2% <ø> (ø)
exporters/stdout/trace.go	65.2% <ø> (ø)
exporters/trace/zipkin/model.go	98.6% <ø> (ø)
exporters/trace/zipkin/zipkin.go	72.9% <ø> (ø)
sdk/trace/provider.go	83.9% <ø> (ø)
... and 6 more