add ssh-agent option to set default lifetime of identities

By default, ssh-agent stores identities forever. It has an option to
set a maximum lifetime for identites (useful to expire passphrase protected
keys). Allow this option to be set using:

zstyle :omz:plugins:ssh-agent lifetime <time>
Merge pull request #1664 from marcel-wolf/master

add ssh-agent option to set default lifetime for identities