add config for snap

[tg123]

#190

current supported snap set sshpiperd

missing doc

sshpiperd.plugins

workingdir.root
workingdir.allow-baduser-name
workingdir.no-check-perm
workingdir.strict-hostkey
workingdir.no-password-auth
yaml.config
yaml.no-check-perm
fixed.target
failtoban.max-failures
failtoban.ban-duration
sshpiperd.address
sshpiperd.port
sshpiperd.server-key
sshpiperd.server-key-data
sshpiperd.server-key-generate-mode
sshpiperd.login-grace-time
sshpiperd.log-level
sshpiperd.typescript-log-dir
sshpiperd.banner-text
sshpiperd.banner-file
sshpiperd.drop-hostkeys-message