x.vweb: add cors middleware

[Casper64]

fix for #20708

This pr adds a middleware function in x.vweb to handle CORS requests.
It also includes a small test and a modified example of examples/vweb/cors

CORS request can be traced by adding -d vweb_trace_cors when you compile

Usage

You only need to add vweb.cors as middleware to your app and configure CORS

Example:

import x.vweb

pub struct Context {
	vweb.Context
}

pub struct App {
	vweb.Middleware[Context]
}
fn main() {
	mut app := &App{}
	// use the cors middleware
	app.use(vweb.cors[Context](vweb.CorsOptions{
		origin: '*'
		allowed_methods: [.get, .head, .put, .post, .patch, .delete]
	}))

	vweb.run[App, Context](mut app, 8080)
}

[enghitalo]

How will multiple origin cors works?

[Casper64]

How will multiple origin cors works?

Good suggestion, I can make the origin into an array.

From the MDN docs:

Only a single origin can be specified. If the server supports clients from multiple origins, it must return the origin for the specific client making the request.

[xqliang]

Yes should allow a list of hosts, and return the requested origin(even when the allowed host is *).

[spytheman]

@Casper64 , please run git pull --rebase origin master .

[spytheman]

Sorry, it was easier for me to do it.