Out of box support for retrieving configuration properties from environment variables and system properties

As an application developer or deployer or tester or framework developer or tester, I expect to be able to use system properties and environment variables to easily override configuration values so that I can easily make necessary adjustments to the (application and/or framework) configuration at deployment or testing time.

Source: https://github.com/eclipse-ee4j/config/discussions/27#discussioncomment-1809272

Start by reading the linked discussion, which provides the stated context for this request. Determine the configuration API and behavior needed for system properties and environment variables to override configuration values, then verify that application, deployment, testing, and framework use cases are covered.

Written by the indexing model from the issue text.