[trace] add Unmarshaler functionality to SpanContext and subfields

[jackgopack4]

Adds Unmarshaler interface implementations for trace.SpanContext and subfields.

This closes #1819 and also will enable less custom code to close open-telemetry/opentelemetry-collector#11740 and open-telemetry/opentelemetry-collector#12212.

Unit tests added for new functionality.

[dmathieu]

This should close #1819

[pellared]

As far as I remember, we never had an agreement that we should add support for unmarshaling.

[jackgopack4]

As far as I remember, we never had an agreement that we should add support for unmarshaling.

Thanks for this context; I work more in the collector than in the library so I was not aware of previous discussions regarding this matter. I just didn't understand why the data would support marshaling but not unmarshaling.

Additionally, it would help us with collector stability, specifically the observability requirements as the exporters create an internal span for tracking progress of requests through the pipeline via the trace.SpanContext object, which is lost if the traces are sent to the persistent queue via file storage extension or similar.

We should be able to work around this by implementing custom structs and helpers to imitate the unmarshal function, but it made sense to me to submit the PR for review in the library as well.

[pellared]

@jackgopack4, just to be clear. Ths was not a complain towards your work. Some of the conversation can be found here #1819. We appreciate your willingness to contribute.

I remember we did not want to add capabilities into the SDK that are not defined in the specification and could be a problem in future e.g. when unmarshaling from JSON feature is defined in the specification. The fact that we have MarshalJSON in these exported types we consider as a legacy.

[jackgopack4]

thank you. If spec compliance is the concern, I've updated the JSON tags for the objects in this PR to match the OTel API Specification and added another note to the CHANGELOG to reflect it

[jackgopack4]

@MrAlias can you remind me what comments you want added to the trace.MarshalJSON function to land this change?