Implement the aspects discussed under the "Runtime" section of https://community.jboss.org/docs/DOC-52712

Add an API to the OperationContext for handlers to publish capabilities and for other handlers to register a requirement for those capabilities and to access the API object associated with the capability.

The registry of capabilities and requirements should be maintained with a semantic equivalent to the resource tree. The registry is copied-on-write, making the copy invisible to concurrently executing operations, and then the copy is published on commit of the operation that modified it. If the operation does not commit, the copy is discarded, so handlers have no need to revert changes they make to the registry.