Single header B3 support

[mwear]

Is your feature request related to a problem? Please describe.

OpenTelemetry JS does not have single header support for B3. Other languages do:

• go
• python
• java

This is likely to become a requirement via: open-telemetry/opentelemetry-specification#960

Describe the solution you'd like

The extract method for the B3 propagator should handle the single header b3 case.

[mwear]

I can start working on this. Let me know if there are any concerns about the idea, or approach.

[mwear]

As I was digging into this, I realized there is quite a bit of variation in how different implementations handle the b3 trace flags and parent span id propagation. I opened this issue: open-telemetry/opentelemetry-specification#1004 as a result.