Support non root index in ContentMirrorStoreStrategy

So far property indexes have been only supported for root indexes i.e. index defined under /oak:index. With OAK-6535 its possible to enable sync support for non root indexes.

From impl ContentMirrorStoreStrategy supports specifying a pathPrefix. However there is no way to set this from outside.

We should provide a way to set this and see if non root indexes can be supported