Make environment variable propagation spec compliant

[pellared]

Is your feature request related to a problem?

Towards: open-telemetry/opentelemetry-specification#5040

The current implementation of environment variable propagation carrier is not specification compliant: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/context/env-carriers.md

At quick glance:

• the name normalization is different
• the env var names read from the OS are not normalized when getting the value
• caching could be done only once

Additional context

Follows #3817

OTel .NET, Go, Java implementation should be already specification compliant.

_{Tip: React with 👍 to help prioritize this issue. Please use comments to provide useful context, avoiding +1 or me too, to help us triage it. Learn more here.}

[pellared]

CC @marcalff

[pellared]

Related spec PR that should help with the implementation open-telemetry/opentelemetry-specification#5102