implement rooms and not_rooms filters on /message

I really doubt anybody is sending /message requests with a filter that
rejects the entire request, but it's the first step in the filter
implementation.

src/utils.rs

@@ -1,4 +1,5 @@
 pub(crate) mod error;
+pub(crate) mod filter;
 
 use std::{
     borrow::Cow,