Terminal: SSH to NAS host instead of container shell

- Rewrite terminal.py to use asyncssh instead of pty/fork
- Add SSH key pair for dashboard container auth
- Mount SSH key and add SSH config vars in docker-compose
- Install openssh-client in Dockerfile
- Add asyncssh to requirements.txt

dashboard/ssh/dashboard_terminal

@@ -0,0 +1,7 @@
+-----BEGIN OPENSSH PRIVATE KEY-----
+b3BlbnNzaC1rZXktdjEAAAAABG5vbmUAAAAEbm9uZQAAAAAAAAABAAAAMwAAAAtzc2gtZW
+QyNTUxOQAAACAA1B82gNubHN9YSbouzZBx5ZcBAfSXrTEf6j2+k9p/MwAAAJi8dYFQvHWB
+UAAAAAtzc2gtZWQyNTUxOQAAACAA1B82gNubHN9YSbouzZBx5ZcBAfSXrTEf6j2+k9p/Mw
+AAAEANwPXx/fEISyOhps72XCb5Okusk9wHUTW6TjHuqfKAEQDUHzaA25sc31hJui7NkHHl
+lwEB9JetMR/qPb6T2n8zAAAAEmRhc2hib2FyZC10ZXJtaW5hbAECAw==
+-----END OPENSSH PRIVATE KEY-----

dashboard/ssh/dashboard_terminal.pub

@@ -0,0 +1 @@
+ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIADUHzaA25sc31hJui7NkHHllwEB9JetMR/qPb6T2n8z dashboard-terminal