Include RewriteRequestHeader filter similar to RewriteResponseHeader

When downstream systems have similar response headers resulting in persistent browser state, a solution could be to simply modify their value to make them unique (e.g. Set-Cookie). If you do so you need to do the reverse operation for request headers representing the same value (like Cookie) which is currently not available as a default filter.

A RewriteRequestHeaderGatewayFilterFactory similar to the existing RewriteResponseHeaderGatewayFilterFactory (equal Config) would solve the problem. Currently I wrote such a filter myself as a work around. As this filter is a reverse one of the existing one I would recommend to add it as a default functionality.

If approved I could help with a pull request for the functionality.