Add configuration option to support link rewriting for Plain rest services

The use case is a Rest Plain mount that does not use sitemap items. HST now creates rest links for individual items in the format "rest mount path" + "repository path to item". To make this link work developers have to create resources that match these default links.

The other option is to add configuration so that HST can write links for items that are available through user defined URIs. Maybe by injecting this info using Spring.

For example: a news item is available in the service under /site/rest/news/item/path-to-item. The /item segment should be part of the URL that is created and should be used by HST to write the correct link.