feat: add support for email triggers

[Schniz]

What this PR solves / how to test:

Ain't much but it's honest work ;D
This PR adds support for email handlers in Cloudflare Workers:

export default {
  async email(message, env, ctx) {
    // this is running in a trace!
  },
};

This PR does not instrument email.reply etc. It's out of scope of this PR. It's only for creating a span/trace for receiving the emails.

[jahands]

SO excited for this!!

[evanderkoogh]

Oh this is great! And I have completely missed it.. Thanks Github for letting me know!

I'll try to get this merged over the next couple of days.. cheers!

[evanderkoogh]

I am so sorry it took so long to get around to this, but I have merged it :)

Thanks for this.. and never let perfection stand in the way of making progress.. in this case, email.reply is an entirely different can of worms. Just getting a span for an email coming in is already infinitely better than not having any spans :)