envcar: add environment carrier

[Joibel]

Fixes #8300

Add a carrier implementing environment carrier based on top of TextMapCarrier.

Allows the callers selection of mechanism for setting environment variables, compatible with os.Setenv - for users who'd like to set environment variables in spawned processes.

This is mostly a migration of @pellared's work in open-telemetry/opentelemetry-go#6778 to go-contrib. I've added further test coverage to appease the coverage bot.

There are three example tests for how to use it.

This is incomplete due to ownership: I'd be happy to be CODEOWNER, or add @pellared as the owner.
I've set myself as CODEOWNER

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.1%. Comparing base (69addb4) to head (517f352).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #8442   +/-   ##
=====================================
  Coverage   82.1%   82.1%           
=====================================
  Files        180     182    +2     
  Lines      13722   13756   +34     
=====================================
+ Hits       11270   11304   +34     
  Misses      2049    2049           
  Partials     403     403           

Files with missing lines	Coverage Δ
propagators/envcar/carrier.go	100.0% <100.0%> (ø)
propagators/envcar/version.go	100.0% <100.0%> (ø)

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[kamphaus]

I see that other propagators have example_test.go files.
Would it be interesting to add such for envcar?

I think two scenarios might be interesting:

• once with os.Setenv
• once to show how to launch a child process with different context than the current process

[pellared]

Let's please add a test that ensures that it works correctly in a scenario when the application (code) spawns processes from a few concurrent goroutines and each process is created inside a new span (so that each of the new process receives unique trace context).

Reference: https://opentelemetry.io/docs/specs/otel/context/env-carriers/#process-spawning

[pellared]

Few nit comments.

I am no longer blocking this PR. Only a few cleanups are needed.

[pellared]

@kamphaus, @adrielp, can you please take another look?

[pellared]

Minor feedback regarding docs. Besides, LGTM. Great work!

[dmathieu]

I've invited you to the @open-telemetry/go-contributors team. This is required to let GH actions auto-request reviews from PRs opened against this module.