Store project's ssh keys in DB

[Egor-S]

Closes #652

• Add SSH key pair to project model in DB
• Add migration for SSH keys generation
• Use the project's ssh key for gateway creation and service publication
• LambdaLabs still uses hub_ssh_key

Script for coping existing keys to the database:

cd ~/.dstack/server
export PROJECT=main
export PRIVATE_KEY=$(cat ssh/hub_ssh_key)
export PUBLIC_KEY=$(cat ssh/hub_ssh_key.pub)
sqlite3 data/sqlite.db "UPDATE projects SET ssh_private_key='$PRIVATE_KEY', ssh_public_key='$PUBLIC_KEY' WHERE name='$PROJECT'"