Feature request: multiple external PSKs

[sshock]

Hi, I know the TLS 1.3 protocol supports multiple PSK in the pre_shared_key extension.

It would be neat if picotls also supported multiple PSKs, e.g., by changing the pre_shared_key in the st_ptls_context_t to be a list. (Then client side pre_shared_key extension would include multiple OfferedPsks, and server side could test and select the first one that works.)

Thanks!