trace: spec compliant trace and span ids

[jtescher]

The spec only defines conversions between ids and hex/binary values. This updates the TraceId and SpanId APIs to be compliant by removing explicit conversions from u128/u64 (except for test convenience methods) and adding methods for converting to/from byte arrays and hex strings.

[codecov]

Codecov Report

Merging #689 (a697fa6) into main (28d2f28) will decrease coverage by 0.17%.
The diff coverage is 83.72%.

@@            Coverage Diff             @@
##             main     #689      +/-   ##
==========================================
- Coverage   63.73%   63.56%   -0.18%     
==========================================
  Files          96       96              
  Lines        7708     7715       +7     
==========================================
- Hits         4913     4904       -9     
- Misses       2795     2811      +16     

Impacted Files	Coverage Δ
opentelemetry-jaeger/src/exporter/mod.rs	58.43% <0.00%> (-0.29%)	⬇️
opentelemetry/src/sdk/trace/tracer.rs	90.76% <ø> (-0.05%)	⬇️
opentelemetry/src/trace/noop.rs	75.00% <ø> (-0.52%)	⬇️
opentelemetry-datadog/src/exporter/model/v03.rs	87.03% <66.66%> (-4.63%)	⬇️
opentelemetry-datadog/src/exporter/model/v05.rs	86.44% <66.66%> (-4.13%)	⬇️
opentelemetry/src/trace/span_context.rs	89.54% <90.47%> (-2.04%)	⬇️
opentelemetry-aws/src/lib.rs	90.50% <100.00%> (-0.58%)	⬇️
opentelemetry-datadog/src/exporter/mod.rs	12.06% <100.00%> (ø)
opentelemetry-datadog/src/lib.rs	91.30% <100.00%> (-0.08%)	⬇️
opentelemetry-jaeger/src/lib.rs	92.55% <100.00%> (-0.08%)	⬇️
... and 10 more

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[djc]

Some minor concerns, but this mostly looks good. I like having INVALID as a const.