feat(core): Add tunnel server helper

[timfish]

Closes #13453 and gives a simpler path for #10309 and adding tunnel support to other frameworks.

The current tunnel troubleshooting docs suggest this code but it doesn't work with binary envelopes so breaks with compressed replay envelopes and attachments.

const SENTRY_HOST = "oXXXXXX.ingest.sentry.io";
const SENTRY_PROJECT_IDS = ["123456"];

export const postAction = async ({ request }) => {
  try {
    const envelopeBytes = await req.arrayBuffer();
    const envelope = new TextDecoder().decode(envelopeBytes);
    const piece = envelope.split("\n")[0];
    const header = JSON.parse(piece);
    const dsn = new URL(header["dsn"]);
    const project_id = dsn.pathname?.replace("/", "");

    if (dsn.hostname !== SENTRY_HOST) {
      throw new Error(`Invalid sentry hostname: ${dsn.hostname}`);
    }

    if (!project_id || !SENTRY_PROJECT_IDS.includes(project_id)) {
      throw new Error(`Invalid sentry project id: ${project_id}`);
    }

    const upstream_sentry_url = `https://${SENTRY_HOST}/api/${project_id}/envelope/`;
    await fetch(upstream_sentry_url, {
      method: "POST",
      body: envelopeBytes,
    });

    return json({}, { status: 200 });
  } catch (e) {
    console.error("error tunneling to sentry", e);
    return json({ error: "error tunneling to sentry" }, { status: 500 });
  }
};

Rather than get users to parse their own envelopes for this info, this PR adds a handleTunnelEnvelope function to @sentry/core which handles all the logic for forwarding envelopes.

If your client DSN is the same as your server DSN, the above example code becomes:

export const postAction = async ({ request }) => {
  try {
    await handleTunnelEnvelope(await request.arrayBuffer());
    return json({}, { status: 200 });
  } catch (e) {
    console.error("error tunneling to sentry", e);
    return json({ error: "error tunneling to sentry" }, { status: 500 });
  }
};

If you want to forward envelopes for other DSNs, you can just pass an array as dsnAllowList in the options. Not sure about this naming. Is allowedDsns better?

We can further wrap the handleTunnelEnvelope function and expose more complete examples for each framework. For example we could expose a helper which takes a Request and returns a Response or export an API route ready to use.

[mydea]

I like it! IMHO it is not that easy to handle envelopes (since replay), so it's nice if we can provide this to users instead of asking everybody to re-implement this.

[timfish]

The envelope parsing utils are also all in @sentry/utils which isn't part of our public API and could technically change between minor versions.