Envoy lightstep option propagation_modes not supported

[rbtcollins]

Please describe your use case / problem.

The default in Envoy for tracing propagation headers is X-Ot-Span-Context. Using Envoy directly it is possible to change to different propagation_modes. We'd like to standardise on one of the standardised carriers defined in opentelemetry: for instance (and specifically for us) the W3C TraceContext headers - https://opentelemetry.io/docs/go/instrumentation/#propagators-and-context - e.g. OTEL_PROPAGATORS="tracecontext" in https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/sdk-environment-variables.md

https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/context/api-propagators.md#global-propagators specifies a default of TraceContext + Baggage.

Currently we pay a cost of integration because Emissary's default is different to that in language toolkits and frameworks.

Describe the solution you'd like
Support the Envoy API for changing propagation method. Supporting the set-of-several approach that the underlying API supports would permit us to gracefully migrate across without a flag moment.

Describe alternatives you've considered
Shifting Emissary's default to be TraceContext for tracing drivers that support it would be nice too, but isn't the core concern.

Additional context
Add any other context about the feature request here.