CLI - Print when using an unstable command

[bfops]

No description provided.

[gefjon]

Requires clarification: which commands are unstable?

At least describe, but presumably others.

Should print approx.:

The spacetime {subcommand} command is experimental and subject to change.

to stderr (not stdout!) at the start of invocation, before doing whatever it is the command does.

[gefjon]

Should likely also add unstable markers to the --help text of the unstable commands.

[bfops]

Unstable commands should be listed in the CLI proposal (https://github.com/clockworklabs/SpacetimeDBPrivate/blob/master/proposals/0026-cli.md). It should be mostly up-to-date, but there could be some stuff missing.

[bfops]

ah I found the other ticket I made for this: #1820