Add an API to Nosecone to get the nonce

[davidmytton]

This would help when developers need to provide the nonce to scripts which do not use something like the Next.js Script component.

For example, PostHog requires you to pass the nonce into the init code.

[blaine-arcjet]

The Next.js <Script> component auto-injects the nonce.

We can still add something for PostHog.

[blaine-arcjet]

I dug into this a bit and found that while Next.js provides a context into middleware, it doesn't propagate past the middleware execution, so I don't see a way to provide this. See also: vercel/next.js#67305

There's a Next.js guide where they put it in the request header but that seems like a footgun because then it could be spoofed by attackers if they can figure out when middleware isn't executed.

[blaine-arcjet]

I still need to understand the instrument.js file because that's supposedly usable for propagating a context but it didn't seem to...