Handler support for l4 plugin

[eth-limo]

Hi @RussellLuo ,

Is it possible to use your other handlers with the l4 plugin?

[RussellLuo]

Hi @eth-limo, all handlers except layer4 in this repo are expected to be used at the HTTP level.

If you want to rate limit connections by IP at the TCP level, I guess we need to implement a layer4-specific rate_limit handler.