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!

I'm thinking that it might be better to have a more generic callback mechanism that doesn't mix the concept of "authorization" in.  For example:

```clj
  (if-let [rewrite-uri-callback-fn (:rewrite-uri-callback-fn options)]
           (rewrite-uri-callback-fn target-uri req)
           target-uri)
```

The implementation of the "rewrite-uri-callback-fn", then, could do the authorization check and return the original URI or redirect URI -- or do something totally different to derive an alternative URI.  This approach would make the callback mechanism for this method more similar to the generic one that the customizeProxyRequest() method uses.