[ssh_x509] support pkcs#11 ed25519 keys, status and plan

[ssh_x509 at roumenpetrov.info]

Hello,

Release 16.2 (25 Jun 2025) supports "provider managed identities". 
Indirectly, this also means support for pkcs#11 ed25519 keys. Just 
pushed commit that tests this functionality. It seems to me it works 
just fine. So, provider as loadable module works unlike various pkcs#11 
engines. Remark: this was main reason to force direct support for EC 
keys and certificates stored in PKCS#11 tokens in version 8.8 (29 Feb 
2016).

The support is direct i.e., identity should use prefix "store:", 
followed by provider URI. For instance identity 
'store:pkcs11:id=%%00%%01' refer to key with identifier '0001'.

Support in ssh-agent is with normal priority. It will be reimplemented 
with planed migration to OpenSSL 3x API. Preferred is use of PQ 
algorithms from cryptographic library.


Regards,
Roumen Petrov