Add an EnvVarPropagator decorator with example code for the env context specification

[adrielp]

What are you trying to achieve?

This relates to this comment thread on issue pull request #4454.

The goal is to add some form of decorator, example code (or preferably both) for SDK and other implementations to follow.

• [x] The decorator should be called EnvVarPropagotor which decorates a TextMapPropagator in the context of environment variables. (there are other optional names that may be considered)
• [x] The example code should behave according to the specification; see the python and swift examples in the comment thread.

[mx-psi]

From triage session on 2025-04-07 we think this is ready based on previous discussion including the comment thread linked above. @reyang could be a sponsor if needed, but this seems like a non-controversial change that can be done without a sponsor.

[adrielp]

Opened a pull request which is ready for initial review to resolve this.

[adrielp]

Supplementary guidance has been added. Next pieces of work will be part of phase two proposal and include library support.