context, vweb: add ability to set and get values on vweb.Context (#18564)

vlib/vweb/tests/middleware_test.v

@@ -239,6 +239,13 @@ fn test_redirect_middleware() {
 	assert received.ends_with('302 Found')
 }
 
+// Context's
+
+fn test_middleware_with_context() {
+	x := http.get('http://${localserver}/with-context') or { panic(err) }
+	assert x.body == 'b'
+}
+
 fn testsuite_end() {
 	// This test is guaranteed to be called last.
 	// It sends a request to the server to shutdown.

vlib/vweb/tests/middleware_test_server.v

@@ -46,6 +46,7 @@ fn main() {
 			'/admin/':        [middleware1]
 			'/other/':        [middleware1, middleware2]
 			'/redirect':      [middleware_redirect]
+			'/with-context':  [context1]
 		}
 	}
 	eprintln('>> webserver: pid: ${os.getpid()}, started on http://localhost:${http_port}/ , with maximum runtime of ${app.timeout} milliseconds.')
@@ -220,6 +221,12 @@ pub fn (mut app App) redirect_route() vweb.Result {
 	return app.text('${result}should_never_reach!')
 }
 
+['/with-context']
+pub fn (mut app App) with_context() vweb.Result {
+	a := app.get_value[string]('a') or { 'none' }
+	return app.text(a)
+}
+
 // middleware functions:
 
 pub fn (mut app App) before_request() {
@@ -256,6 +263,11 @@ fn middleware_redirect(mut ctx vweb.Context) bool {
 	return false
 }
 
+fn context1(mut ctx vweb.Context) bool {
+	ctx.set_value('a', 'b')
+	return true
+}
+
 // utility functions:
 
 pub fn (mut app App) shutdown() vweb.Result {