How to support system property substitution in file config

[jack-berg]

Do you think it'd make sense to add sth like ${sys:otel.traces.exporter} for system properties as well?

Originally posted by @mateuszrzeszutek in https://github.com/open-telemetry/opentelemetry-java/pull/5914#pullrequestreview-1684562062

Environment variable substitution in file configuration is modeled after the collector, which doesn't have system properties.

System properties are very popular / common in java. How do we support substitution of system properties while keeping with the spirit of collector environment variable substitution?