feat: Workspace Proxy picker show latency to each proxy (#7486)

* chore: Add cors to workspace proxies to allow for latency checks
* Add latency check to wsproxy

Use performance API timings.
- Fix cors and timing headers
- Accept custom headers

coderd/latencycheck.go

@@ -0,0 +1,24 @@
+package coderd
+
+import (
+	"net/http"
+	"net/url"
+	"strings"
+)
+
+func LatencyCheck(allowedOrigins ...*url.URL) http.HandlerFunc {
+	allowed := make([]string, 0, len(allowedOrigins))
+	for _, origin := range allowedOrigins {
+		// Allow the origin without a path
+		tmp := *origin
+		tmp.Path = ""
+		allowed = append(allowed, strings.TrimSuffix(origin.String(), "/"))
+	}
+	origins := strings.Join(allowed, ",")
+	return func(rw http.ResponseWriter, r *http.Request) {
+		// Allowing timing information to be shared. This allows the browser
+		// to exclude TLS handshake timing.
+		rw.Header().Set("Timing-Allow-Origin", origins)
+		rw.WriteHeader(http.StatusOK)
+	}
+}