These all start with ssh-agency-:

• bin-dir (where are my ssh commands)
• add-executable (ssh-add command)
• agent-executable (ssh-agent command)
• env-file (write SSH agent environment variables so that other processes can find it)
• private-key-p (why?)
• keys (a regex of key files to add)

Again, these start with ssh-agency-:

• add-keys
• start-agent (also write some environment variables to help us find it)
• find-agent
• status
• ensure (start and add keys if needed)

The autoload mechanism hooks ensure into all the magit functions.

But how do I include the package?