(TK-21) Add `:callback-fn` option to `add-proxy-route`

This commit adds a `:callback-fn` option to `add-proxy-route`'s
`proxy-options`. This option can be given a function that will manipulate the
request object (for example, to add a header) when using Jetty's proxy
servlet.
(TK-21) Update README for `add-proxy-route` `:callback-fn` option