Update codes to match specification

[MrAlias]

• Update the codes package to match the changes to the specification: Error Flagging with Status Codes opentelemetry-specification#966
• Add a String, UnmarshalJSON, and MarshalJSON method to the codes.Code struct.
• Update the sdk/export/trace.SpanData to use the otel/codes package as there now isn't a 1-to-1 conversion to the gRPC codes and the downstream OTLP is also changing to these codes: Modify Span status according to specification opentelemetry-proto#224

Resolves #1213

[codecov]

Codecov Report

Merging #1214 into master will decrease coverage by 0.0%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #1214     +/-   ##
========================================
- Coverage    76.7%   76.6%   -0.1%     
========================================
  Files         133     134      +1     
  Lines        5809    5807      -2     
========================================
- Hits         4456    4449      -7     
- Misses       1104    1109      +5     
  Partials      249     249             

Impacted Files	Coverage Δ
api/apitest/harness.go	100.0% <ø> (ø)
api/trace/tracetest/span.go	96.0% <ø> (-1.3%)	⬇️
bridge/opentracing/bridge.go	40.0% <ø> (ø)
bridge/opentracing/internal/mock.go	70.4% <ø> (-0.7%)	⬇️
codes/codes.go	100.0% <ø> (ø)
exporters/otlp/internal/transform/span.go	98.0% <ø> (+0.1%)	⬆️
exporters/trace/jaeger/jaeger.go	84.6% <ø> (ø)
sdk/trace/span.go	91.6% <ø> (-1.5%)	⬇️
semconv/http.go	95.2% <ø> (+<0.1%)	⬆️
... and 20 more