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)))

There is no automated test, as there's additional complexity in ensuring that the shield POP has also gone up in time for the test to run. We could potentially add one that continually sends a request to the shield until it responds, but for now I've just tested this manually, which is also what, e.g. the Go SDK does.

[zkat]

doesn't FASTLY_HOST_ERROR_BUFFER_LEN usually set buf_size to the expected buf_size, so you don't have to keep retrying like this?

[TartanLlama]

Usually, yes, not in the case of this host call it seems. I'm following the Rust SDK behaviour fwiw

[TartanLlama]

https://github.com/fastly/fst-compute-sdk-rs/blob/fe621bae505f843d55fa99afae186f453c847aee/fastly/src/shielding.rs#L127

[zkat]

apparently it doesn't. Absolutely rude.