Config: support all resources on classpath

[tomas-langer]

Current implementation of config sources for classpath resources only uses the first resource found.
We should support (both from meta-config and from a builder) adding config source for each instance of a resource on a classpath.
Usage:
microprofile-config.properties already requires this approach. We can do this using a builder through ConfigSources.classpathAll("META-INF/microprofile-config.properties"), we must be able to do the same thing through meta-config

Example meta-config (an option how to implement):

sources:
  - type: "multi"
     source-type: "classpath"
     properties:
       resource: "microprofile-config.properties"

The multi would tell meta configuration that this would return multiple instances of a config source.
The source-type would point to a named ConfigSourceProvider.
This would probably require additional method in ConfigSourceProvider:
List<ConfigSource> createMulti(String type, Config metaConfig)

[tomas-langer]

This is now working.
When using meta configuration, use multi-source: true to mark the source as a multiple source.
When using a builder, there are new methods on ClasspathConfigSource:

1. createAll(Config) to load all sources from meta configuration
2. createAll(String) to load all resource instances from the classpath

Resolved by #1417