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

Add 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.
Purely cosmetic -- we try to align columns on our keys and values in our map definitions.  So, `:target` would start at the same column as `:proxy` would -- like what you did on L167 above.