[SDK] Support OTEL_SDK_DISABLED environment variable

[marcalff]

The SDK spec recently added a new environment variable, OTEL_SDK_DISABLED

See open-telemetry/opentelemetry-specification#2679

Implement support for OTEL_SDK_DISABLED, for all signals.

[github-actions]

This issue was marked as stale due to lack of activity.

[github-actions]

This issue is available for anyone to work on. Make sure to reference this issue in your pull request.
✨ Thank you for your contribution! ✨

[RichardChukwu]

Is this available to be worked on @marcalff ?

[divenire05]

If no one is working on this, could I take it up? Feel free to assign it to me. @marcalff