feat: Shielding support

[TartanLlama]

Adds support for shielding. Example:

/// <reference types="@fastly/js-compute" />
    
import { Shield } from "fastly:shielding";

async function app(event) {
    const shield = new Shield('wsi-australia-au');
    // If running on the shield POP, fetch from the origin directly
    if (shield.runningOn()) {
        return await fetch('https://http-me.fastly.com/anything', { backend: 'httpme' });
    }
    // Otherwise, route the request through the shield using an encrypted connection
    return await fetch(event.request, { backend: shield.encryptedBackend() });
}

addEventListener("fetch", (event) => event.respondWith(app(event)))