WIP - TK Proxy fixes - REVIEW ONLY

For now, this is a review-only PR showing what changes are required for the pe-trapperkeeper-proxy.

- This PR
- bumps schema version to support the Bool type (needed in dependent projects)
- moves jetty version to a variable to enable easy jetty version change
- bumps version of the http client
- enables using the latest jetty client (a workaround with the Logger creation is needed)
- adds another hook in the add-proxy-route which enables changing the URI early enough in case of redirection (AFAIK request in the callback-fn cannot change its hostname), if we say that we'll always redirect within the same hostname then we can try to merge this hook into the callback-fn as well
- adds support for authorization via the 'authorize-fn' callback function which can either grant access to the target URI, or send a redirect to a different URI (typically the "forbidden" page)

Note: The documentation is not updated (regarding the add-proxy-route) since this is just a proposal of adding yet another callback function and I am not sure if it's acceptable. If not then we can try to live with the original callback-fn only but we'll be limited in actions that can be done if user is not authorized to visit the target URL. Ideas are welcome!

Is there a reason you went with 0.2.0-SNAPSHOT instead of 0.2.0?  We've tried to keep these on non-SNAPSHOT versions wherever possible.