Improve ignition target selection in faux-mgs

[mkeeter]

Ignition commands in faux-mgs require specifying an "ignition target", which has to be looked up in RFD 144 § Switch port map. This isn't the same as cubby number, which is annoying.

This PR makes it easier to deal with:

Get the ignition state for a port (only valid if the SP is an ignition controller)

Usage: faux-mgs --interface <INTERFACE> ignition [OPTIONS]

Options:
  -t, --target <TARGET>    Ignition target
  -c, --cubby <CUBBY>      Sled cubby (0-31)
  -s, --sidecar <SIDECAR>  Sidecar ('local' or 'peer')
  -p, --psc <PSC>          PSC index (0-1)
  -a, --all                All targets
  -h, --help               Print help

This set of arguments is common to ignition, ignition-link-events, clear-ignition-link-events; ignition-command supports all the flags except --all.

Opening as a draft because racktest uses faux-mgs ignition all here, and we should figure out how to coordinate with its release. We could keep all as a special-cased target (instead of --all), but it's kinda annoying...

[jgallagher]

Does this preclude having a conficts_with_all = ["all"]? Could we get that effect by making IgnitionBulkSelector an enum, or would that force us to have a subcommand to specify the variant?

[mkeeter]

I switched to using argument groups, so you get exactly-one semantics:

Get the ignition state for a port (only valid if the SP is an ignition controller)

Usage: faux-mgs --interface <INTERFACE> ignition <--target <TARGET>|--cubby <CUBBY>|--sidecar <SIDECAR>|--psc <PSC>|--all|COMPAT>

Arguments:
  [COMPAT]  'all' or a target number (backwards-compatibility shim)

Options:
  -t, --target <TARGET>    Ignition target
  -c, --cubby <CUBBY>      Sled cubby (0-31)
  -s, --sidecar <SIDECAR>  Sidecar ('local' or 'peer')
  -p, --psc <PSC>          PSC index (0-1)
  -a, --all                All targets
  -h, --help               Print help

[mkeeter]

After chatting with @jclulow, I restored the support for faux-mgs ignition all and faux-mgs ignition [integer]. The former is used in racktest; I can't find any evidence for the latter being used in scripts.

Due to clap's limitations on optional arguments, I can't support both faux-mgs ignition-command --cubby=0 power-on and faux-mgs ignition-command 0 power-on, so I'm going to break backwards compatibility here. Searching through the entire oxidecomputer organization, there are no uses of ignition-command, ignition-link-events, or clear-ignition-link-events.