feat(client): reload plugin certificates on frpc reload (#5576)

The https2http and https2https plugins read their certificate and key
once when the proxy starts, so a certificate that an external tool renews
on disk is only picked up after the proxy itself is recreated. frpc
reload rebuilt proxies only when their configuration changed, and a
rebuild closes the work connections that are already in use.

Let the plugin own the refresh. The HTTPS plugin server keeps the active
keypair and the last refresh error in one immutable snapshot published
through an atomic pointer, so a TLS handshake only loads that pointer and
never touches the disk. Reload rereads both files, and when their bytes
match the published pair it returns without parsing or replacing
anything; otherwise it parses the pair and swaps the snapshot in. A read
or parse failure keeps the previous certificate serving and records the
error next to it.

frpc reload collects the reloadable plugins of the live proxies and
applies the new configuration inside the same reload transaction that
also serializes control handoff, so a reload that races a reconnect
still refreshes the plugins of the published control. The certificate
files are read only after that transaction has been released. The config
store API keeps applying configuration without refreshing certificates,
because only an explicit reload may reread files. Proxy status reports a
refresh failure next to the still running proxy instead of dropping its
remote address.

No configuration field is added, and frps and the wire protocol are
unchanged. Without a reload nothing changes: certificates are still read
once at startup, generated self-signed certificates still never reload,
and a proxy whose configuration changed still goes through the existing
rebuild path.

client/http/controller.go

@@ -149,7 +149,7 @@ func (c *Controller) buildProxyStatusResp(status *proxy.WorkingStatus) model.Pro
 	}
 	psr.Plugin = baseCfg.Plugin.Type
 
-	if status.Err == "" {
+	if status.Err == "" || status.Phase == proxy.ProxyPhaseRunning {
 		psr.RemoteAddr = status.RemoteAddr
 		if slices.Contains([]string{"tcp", "udp"}, status.Type) {
 			psr.RemoteAddr = c.serverAddr + psr.RemoteAddr