This commit adds a new OAuth "flow" to perform the token exchange protocol described in RFC 8693 [0]. 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 [1]. - Okta uses it for some advanced delegation features [2]. - Some open-source auth tools like Curity seem to support it [3], as do various commercial identity management platforms like Asgardeo [4]. - Posit Connect uses it to power its OAuth integration feature [5]. Closes r-lib#460. [0]: https://datatracker.ietf.org/doc/html/rfc8693 [1]: https://cloud.google.com/iam/docs/workforce-obtaining-short-lived-credentials [2]: https://developer.okta.com/docs/guides/set-up-token-exchange/main/ [3]: https://curity.io/resources/learn/token-exchange-flow/ [4]: https://wso2.com/asgardeo/docs/guides/authentication/configure-token-exchange/ [5]: https://docs.posit.co/connect/admin/integrations/oauth-integrations/ Signed-off-by: Aaron Jacobs <aaron.jacobs@posit.co>

Implement oauth_flow_exchange() #460

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