Add `daemon` or `engine` subcommand

In the past, docker had the daemon built-in as a single binary under the daemon subcommand (or if you go far enough back just the -d flag as I recall).

I propose that we add this subcommand back... only better.
The subcommand would be responsible for setting up the daemon on the client node.

On linux this would be starting a daemon as normal.
If docker is run as unprivileged, it could be responsible for setting up all the rootless stuff.
Perhaps could be rootless by default or even rootless only (since it is the client after all).

On non-native platforms this could also be extended to support other cases such as spinning up a platform-native VM to run the daemon, setup socket access, etc.
But anything that we don't actually support the subcommand would just error out as not supported.

In any case this would also configure a context to connect to the daemon.

Honestly this is not a concrete proposal, just exploring some ideas.

I think having the rootless experience tied in with the CLI would be extremely interesting.

The issue names no files, tests, or entry points; begin by clarifying the daemon or engine subcommand scope, supported platforms, rootless behavior, and context setup. Done would require an agreed concrete proposal and implementation boundaries, which the current issue does not provide.