Proxy middleware: add ModifyRequest function to modify http.Request before proxy

[binhnguyenduc]

The proxy middleware currently does not allow any modification to the request, only ModifyResponse. This PR adds that option so caller can pass a custom function to be applied to requests before it hits reverse proxy.

I have also added passing test.

Please consider merging this as it would allow for more flexibility when proxy with Echo.

Thanks!

[asad-awadia]

@aldas is it possible to modify the proxy target urls at runtime?

[aldas]

I think you are looking for: https://github.com/labstack/echo/blob/4a1ccdfdc520eb90573a97a7d04fd9fc300c1629/middleware/proxy.go#L69 and look how commonBalancer works

[asad-awadia]

@aldas oh so I can use the returned object from NewRoundRobinBalancer to change the proxy targets by calling the appropriate methods?

The interface is already implemented internally? I just have to call addTarget with the right parameter ?