add ffi and rust wrapper for domainmap

[jkarneges]

This makes DomainMap available to Rust, without actually using it anywhere yet. For now the API is minimal, only allowing looking up the log_level of a route by ID.

Normally, DomainMap emits a signal whenever the routes file changes, but this only makes sense if there is a C++-managed event loop in the thread that instantiated it otherwise the queued signals may pile up. To make DomainMap usable in Rust apps that won't have such an event loop, this PR makes it so signals are only emitted when one is detected. An assertion is also added to DeferCall to prevent doing the wrong thing by accident.

Manually tested with the following local change:

--- a/src/connmgr/mod.rs
+++ b/src/connmgr/mod.rs
@@ -473,6 +473,10 @@ impl App {
 pub fn run(config: &Config) -> Result<(), Box<dyn Error>> {
     debug!("starting...");
 
+    let domainmap = crate::proxy::domainmap::DomainMap::new("routes");
+    let params = domainmap.lookup("example").unwrap();
+    info!("example log_level={}", params.log_level);
+
     {
         let a = match App::new(config) {
             Ok(a) => a,

Results:

% cat routes 
* test
example.com,id=example,log_level=1 test

% bin/pushpin-connmgr --log-level 3         
[DEBUG] 2026-04-29 11:24:51.670 [pushpin::connmgr] starting...
[DEBUG] 2026-04-29 11:24:51.670 domainmap thread started
[DEBUG] 2026-04-29 11:24:51.672 routes by domain:
[DEBUG] 2026-04-29 11:24:51.672   (default): test
[DEBUG] 2026-04-29 11:24:51.672   example.com: test
[INFO] 2026-04-29 11:24:51.672 routes loaded with 2 entries
[INFO] 2026-04-29 11:24:51.672 example log_level=1
...
[INFO] 2026-04-29 11:24:54.140 routes file changed, reloading
[DEBUG] 2026-04-29 11:24:55.133 routes by domain:
[DEBUG] 2026-04-29 11:24:55.133   (default): test
[DEBUG] 2026-04-29 11:24:55.133   example.com: test
[INFO] 2026-04-29 11:24:55.133 routes loaded with 2 entries
...
^C[INFO] 2026-04-29 11:25:00.778 stopping...
...
[DEBUG] 2026-04-29 11:25:00.784 [pushpin::connmgr] stopped
[DEBUG] 2026-04-29 11:25:00.785 domainmap thread stopped