Consolidate `*Cmd` logic

[azriel91]

Currently:

• EnsureCmd and CleanCmd delegate to ApplyCmd.
• ApplyCmd and StatesDiscoverCmd both collect outcomes to surface errors as a whole.

What we would like for all commands:

• Ability to use other commands' logic, but all of the *Cmd invocations share one progress output.
• Output type is generic
• Collect errors from each item spec, and return all values and errors as a whole.

Use cases:

• EnsureCmd:

  • Forward iteration through each item.

    Calls state_current, apply_check, apply_exec in sequence.

• CleanCmd

  • Forward iteration through each item.

    Calls try_state_current.

  • Reverse iteration through each item.

    Calls apply_check, apply_exec in sequence.

• StatesDiscoverCmd:

  • Forward iteration through each item.

    Calls try_state_current / try_state_desired.

[azriel91]

Design / implementation issue:

1. ApplyCmd wants to call StatesDiscoverCmd::current
2. Inlining the state current discovery logic into ApplyCmd, and executing concurrently requires an intemediate outcomes rx, which is essentially re-implementing CmdBase.
3. Current logic of StatesDiscoverCmd takes in &mut CmdCtx.
4. For automation logic, StatesDiscoverCmd needs immutable references to the flow and parameter specs.
5. However, CmdCtx and SingleProfileSingleFlowView contain &mut Output, and &mut CmdProgressTracker.
6. This makes it hard to pass the container of flow / parameter information around.
7. Only the top level command needs &mut Output and &mut CmdProgressTracker to set up progress rendering.
8. We could move those two out of the view and into the CmdCtx, so that fn_exec can take in the *View and progress and outcome Senders, and this view can be passed to sub commands.
9. If we don't do that, then every command needs to pass through about 5 separate parameters.

So, we should split Output and CmdProgressTracker out of the view into ScopeViewAndOutput.