Refactor the OpenCensus Bridge

[MrAlias]

• Use the default SDK and new SpanRecorder to test the bridge in the new test package instead of the oteltest package. (Move OpenCensus bridge integration tests to their own module #2165)
• Move implementation to an internal package so the test package has access to conversions and types for testing. (Add OpenCensus bridge internal package #2146)
• Add unit tests for the conversions and types. (Add OpenCensus bridge internal package unit tests #2164)
• Remove the utils package. Instead, move the conversion functions it contained to the main package. (Deprecated the bridge/opencensus/utils package #2166)
• Add documentation about the limitations of the package. (Update OpenCensus bridge docs with limitations #2145)

Part of #2121

[codecov]

Codecov Report

Merging #2144 (3801b4c) into main (d18c135) will increase coverage by 0.1%.
The diff coverage is 0.0%.

@@           Coverage Diff           @@
##            main   #2144     +/-   ##
=======================================
+ Coverage   72.2%   72.3%   +0.1%     
=======================================
  Files        176     175      -1     
  Lines      12088   12088             
=======================================
+ Hits        8731    8745     +14     
+ Misses      3115    3103     -12     
+ Partials     242     240      -2     

Impacted Files	Coverage Δ
bridge/opencensus/bridge.go	0.0% <0.0%> (-100.0%)	⬇️
bridge/opencensus/internal/oc2otel/attributes.go	100.0% <0.0%> (+9.5%)	⬆️
bridge/opencensus/internal/span.go	100.0% <0.0%> (+13.0%)	⬆️
bridge/opencensus/internal/tracer.go	100.0% <0.0%> (+22.7%)	⬆️
...pencensus/internal/oc2otel/tracer_start_options.go	100.0% <0.0%> (+27.7%)	⬆️

[MrAlias]

I'm going to split this up.

[MrAlias]

Slit into the constituent parts mentioned in the description.