Ability to add configuration stores

As an application or framework developer, I expect that there will be a way to provide support for additional configuration store(s) so that I can provide configuration properties from run time environments (such as K8s secrets, vaults, etc.).

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

No implementation files, tests, or entry points are identified in the issue. Read the linked discussion first to understand the intended extension model and which configuration stores are in scope. Done means a defined way for applications or frameworks to provide additional runtime configuration stores, with the supported behavior verified by tests.

Written by the indexing model from the issue text.