Design and API to work with SSH-Agent

There were few requests #276 and #277 that ask for ssh-agent API improvements. Currently, the agent is completely internal to the libssh and we can use it only to do authentication or forward the socket.

While thinking about this more, I think we should have an agent "session" that could be used to query keys from agent and operate on the agent connection. The listing of keys should result in pki objects that could be used in the existing pki functions for authentication or for sshsig without need to change the API specifically for agent and we could get rid of the ssh_userauth_agent() API too.

The idea came from the reading and answering to the following mail thread:

https://archive.libssh.org/libssh/2025-12/0000001.html

More inputs and ideas welcomed.