add max_rpm_per_host

Could you please split this function?
```erlang
%% somewhere in src/utils.erl
-module(utils).

ip_from_cowboy_req(Req) ->
  {{IpAddress, _},_} = cowboy_req:peer(Req),
  inet:ntoa(IpAddress).
```
```erlang
%% This function is boolean and should be idempotent. So it should have only one responsibility (i.e. check for RPM limit)
can_process(Req) ->
   statistics:requests_from_host(utils:ip_from_cowboy_req(Req)) =< erl_proxy_app:config(max_rpm_per_host).
```
So, `request_handler:handle/2` will look like this:
```erlang
handle(Req, State) ->
  statistics:add_request_from_host(utils:ip_from_cowboy_req(Req)),
  CanProcess = can_process(Req),
    {ok, ResultReq} = if
      CanProcess -> usual_request(Req);
      true -> too_many_requests(Req)
    end,
```