Add Vercel binding

[twifkak]

There are two three types of Vercel Functions and I'm not sure of the compare/contrast. One comparison here. There is a community Rust binding (API).

For storage, one can connect to an external DB but perhaps a more config-free approach would be to use edge caching and set up internal HTTP APIs for certs/OCSP.

[twifkak]

OK, Vercel doesn't support custom cache keys, which we need to separate SXG responses (for SXG crawlers) from non-SXG (for browsers and non-SXG crawlers). Judging by this flow diagram we should target Edge Middleware which isn't cached, so that users can still enable caching behind it.

[twifkak]

Perhaps the simplest option is to support some external DB with a free tier.

To clarify the "use edge caching and set up internal HTTP APIs" idea: Request the outcome of the last step. The handler for each step recursively fetches the previous step, and sets long cache headers for successful outcomes.

serve_preset_content would be handled by Vercel Functions, not Middleware, so it could be cached. When handling requests for the following URLs:

• cert_url_dirname prefix: fetch /.well-known/sxg-internal/acme/download_certificate and /.well-known/sxg-internal/ocsp/response. If successful, generate a cert-chain+cbor and serve with a 3.5 day cache lifetime. Else, serve an error response with a short lifetime (1 minute?).
• acme/download_certificate: fetch finalize_signing_request. If successful, perform the next step of ACME and serve the final state with a 45-day cache lifetime. Else, error response with a short lifetime.
• acme/finalize_signing_request: fetch check_challenge_finished. Ditto if successful/error.
• acme/check_challenge_finished: fetch request_challenge_validation. Ditto if successful/error.
• acme/request_challenge_validation: Perform initial step. Ditto if successful/error.
• ocsp/response: Fetch from OCSP responder. If successful, set a 3.5 day cache lifetime. Else, set a short lifetime (10 minutes?).

(We may also want to sign these URLs so external requestors can't forge them.)