Expose ExternalModuleService as WildFly capability

ExternalModuleService is used to create the ExternalModuleSpecService which exposes the ModuleDefinition for any external module dependency.

Currently, ExternalModuleService is used via DUPs to scan the deployment and add external modules. We should expose this service via a Capability so it can be easily accessed via not only by the DUPs, but also other services that could require configure external modules.

This capability should be registered in ServerService, see for example PathManager. It should be something that ExternalModuleService implements, the goal is not to expose the start/stop/getValue methods via the capability.

See this as reference comment.