Add support for RFC 8693 token exchange requests

[atheriel]

This commit adds a new OAuth "flow" to perform the token exchange protocol described in RFC 8693. This is a pretty obscure and advanced OAuth feature, but I thought it would be nice to have some helpers to support it in httr2, anyway.

I'm not aware of that many implementations of this RFC, though there are a few to note:

• GCP uses it for a couple of identity federation features.

• Okta uses it for some advanced delegation features.

• Some open-source auth tools like Curity seem to support it, as do various commercial identity management platforms like Asgardeo.

• Posit Connect uses it to power its OAuth integration feature.

Closes #460.

[atheriel]

I don't have write access to merge, you might have to do so on my behalf.

[hadley]

I also gave you write access since you contribute quite a bit 😄