vweb: add user-agent utility method called ua

vlib/vweb/README.md

@@ -1195,6 +1195,19 @@ fn main() {
 
 ### Others
 
+#### -user_agent
+
+Returns the user-agent from the current user
+
+**Example:**
+
+```v ignore
+pub fn (mut app App) user_agent() vweb.Result {
+    ua := app.user_agent()
+    return app.text('User-Agent: $ua')
+}
+```
+
 #### -ip
 
 Returns the ip address from the current user

vlib/vweb/vweb.v

@@ -1035,6 +1035,11 @@ pub fn (mut ctx Context) host_serve_static(host string, url string, file_path st
 	ctx.static_hosts[url] = host
 }
 
+// user_agent returns the user-agent header for the current client
+pub fn (ctx &Context) user_agent() string {
+	return ctx.req.header.get(.user_agent) or { '' }
+}
+
 // Returns the ip address from the current user
 pub fn (ctx &Context) ip() string {
 	mut ip := ctx.req.header.get(.x_forwarded_for) or { '' }