Add OpenCensus bridge internal package

[MrAlias]

This move is to enable access to internal types in a planned test module (see #2144 for more information).

The only functional change is in the error handling. The global error handler cannot be reset multiple times (#2140) so a local variable is used get around this. Additionally, the conversion of the span start options now returns an error instead of trying to formulate a error with the context of the span it is being called for.

In addition to moving the implementation to an internal package this adds tests for all the conversions and methods.

Unit tests for the conversion functions and types methods will be added in a follow on PR.

[codecov]

Codecov Report

Merging #2146 (aa4ca0f) into main (fcf945a) will decrease coverage by 0.0%.
The diff coverage is 85.4%.

@@           Coverage Diff           @@
##            main   #2146     +/-   ##
=======================================
- Coverage   72.2%   72.2%   -0.1%     
=======================================
  Files        170     176      +6     
  Lines      12076   12088     +12     
=======================================
+ Hits        8724    8731      +7     
- Misses      3111    3115      +4     
- Partials     241     242      +1     

Impacted Files	Coverage Δ
bridge/opencensus/utils/utils.go	50.0% <50.0%> (-50.0%)	⬇️
...pencensus/internal/oc2otel/tracer_start_options.go	72.2% <72.2%> (ø)
bridge/opencensus/internal/tracer.go	77.2% <77.2%> (ø)
bridge/opencensus/internal/span.go	86.9% <86.9%> (ø)
bridge/opencensus/internal/oc2otel/attributes.go	90.4% <90.4%> (ø)
bridge/opencensus/bridge.go	100.0% <100.0%> (+14.8%)	⬆️
bridge/opencensus/internal/oc2otel/span_context.go	100.0% <100.0%> (ø)
bridge/opencensus/internal/otel2oc/span_context.go	100.0% <100.0%> (ø)
... and 4 more

[MrAlias]

I'm going to pull the addition of tests out of this and put it in a follow on PR to accelerate the review of this. This PR will not only block the addition of tests PR, but also blocks the refactor of the integration tests into a test module that uses the default SDK.