Support the environment variable propagation

[adrielp]

With the environment variable context propagation spec being added, we'd like to have its support added to Go. This is part of a larger effort to adding support in each of the currently supported languages.

Two example pull requests have been opened in the past that can be used for ideas in conjunction with the specification.

• [Prototype] propagation: add EnvCarrier opentelemetry-go#6778
• Add EnvironmentCarrier for propagating trace information across processes opentelemetry-go#2931

[pellared]

I suggest moving this issue to Go Contrib and implement it under https://github.com/open-telemetry/opentelemetry-go-contrib/tree/main/propagators since it is not a required propagator.