Allow customizing the static hosts configuration

This allows custom runtimes to provide the hosts configuration in their
own way, like using io_uring instead of the std::fs::File::open() to
read the system hosts file.

crates/resolver/src/async_resolver.rs

@@ -436,6 +436,11 @@ impl<C: DnsHandle<Error = ResolveError>, P: ConnectionProvider<Conn = C>> AsyncR
         .await
     }
 
+    /// Customizes the static hosts used in this resolver.
+    pub fn set_hosts(&mut self, hosts: Option<Hosts>) {
+        self.hosts = hosts.map(|hosts| Arc::new(hosts));
+    }
+
     lookup_fn!(
         reverse_lookup,
         lookup::ReverseLookup,