Add certificate selection callback

[mpg]

Suggested enhancement

Add a new callback (server-side) that's called at the right time for the server to select a certificate.

Constraints:

• must be after all ClientHello extensions have been parsed, at the server might use information from those extensions in order to select a certificate
• must be before the ServerHello is sent, as the selection of the ciphersuite depends on the type of certificate selected.

Justification

Mbed TLS needs this because sometimes the server needs to adjust its behaviour based on multiple extensions, so per-extension callback (such as the existing SNI callback) are not enough. An example is servers who want to support the ACME "tls-alpn-01" challenge, where certificate selection needs to happen based not only on SNI but also ALPN extension, as reported in #5331.

Finally, it appears there's a similar callback in OpenSSL since 1.0.2 and people are using it.

[gstrauss]

#5454 "server certificate selection callback" submitted as a Draft for discussion.

[mpg]

Renaming to "add certificate selection callback" - after discussion with the team, we agreed that new callbacks should not be named after a particular point in the handshake, but after the action they're meant to allow. Handshake flows might change between versions and it's our job to shield users from those low-level details and just run the callback at the right time for the associated action.