Add assistance for Liberty-specific MicroProfile ConfigSources, properties and rules (for app-specific vs server-wide config)

[scottkurz]

There are three (somewhat overlapping) ways in which Liberty introduces a Liberty-specific need for validation of MP config properties.

1. Liberty-specific ConfigSources

(server.env, jvm.options, server.xml variables)

2. Liberty-specific properties

E.g.mp.openapi.extensions.liberty.merged.exclude/include from https://openliberty.io/docs/latest/microprofile-config-properties.html

3. Liberty-specific rules

This has just recently been raised as design issue 25327, from UFO discussion of this. There's a tension between the fact that certain properties logically make sense only at a server/JVM-wide scope and the definition of these properties in application-specific locations, like src/main/resources/META-INF/microprofile-config.properties (though if there's only one app per server this conflict resolves itself).

But as discussed in this issue there is a desire to at least warn or possibly prevent users from using app-scoped locations for server-scoped settings. (NOTE: The comment actually starts with the idea that the LMP/LGP would be enhanced with some function along these lines, but we can start with the issue here.)

NOTE:

Though in some ways this issue might seem better suited for another home, this repo already hosts both its own LS impl (for bootstrap.properties and server.env) and an extension to another LS.

This could result in LMP/LGP issues and who knows, possibly even some kind of LSP4MP extension?