Support `--download-only` for `bootc switch`

[jlebon]

For parity with upgrade and because there's legitimate demand for it.

#1320 (comment)

@jlebon is there any chance to also allow this flag for bootc-switch?
Working on a project where we are currently using bootc-switch rather than bootc-upgrade as we need to be able switch to exact versions, rather than relying on the latest available.
At the moment we download the images outside of bootc and then use bootc switch --transport oci. Having the --download-only flag would make this more streamlined.

[jlebon]

I can probably look at this once I get closer to being blocked on it.