Implement Logger.is_enabled

[lmolkova]

Spec defines basic Logger.Enabled method which is now can be enabled/disabled per scope with config.

Filtering by severity and sampling are coming via open-telemetry/opentelemetry-specification#4290.

Having this API now (even before filtering/sampling is finalized in the spec) would help instrumentation to avoid doing potentially expensive operations when logging is not even configured, but also they can start leveraging is_enabled now and don't need to change when SDK starts supporting all the interesting cases behind it.

Related to open-telemetry/opentelemetry-python-contrib#3226 (review)