[carriers] add env carrier

[adrielp]

Description

Supports: #4829

Adds a prototype for the environment variable carrier, specifically decorating Getter and Setter instead of creating a dedicated propagator. The comes from the conversations within the environment variable context propagation specification pull request.

There's a corresponding thread in Slack around this topic.

The example use of the Getter portion of this prototype can be found in this gist.

Type of change

Please delete options that are not relevant.

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x] This change requires a documentation update

How Has This Been Tested?

The Getter was tested by using the prototype in a real environment. Unit tests need to be added after conversation around this prototype.

• [ ] Test A

Does This PR Require a Contrib Repo Change?

• [ ] Yes. - Link to PR:
• [x] No.
• [ ] Unsure

Checklist:

• [x] Followed the style guidelines of this project
• [x] Changelogs have been updated
• [x] Unit tests have been added
• [x] Documentation has been updated