add a `finalCh9DialogKey` attr to Badeline Boost

### I am planning on implementing this myself

Yes

### Describe your request

currently, you can only choose whether or not `CH9_LAST_BOOST` plays, not what dialog key is used.
it would be helpful to add an optional attr to `BadelineBoost`, defaulting to `CH9_LAST_BOOST`, to change that key.

- add fields to `BadelineBoost`, `CS10_FinalLaunch`
- ilhook to change:
```diff
ldarg.0
- ldstr "CH9_LAST_BOOST"
+ ldfld string Celeste.CS10_FinalLaunch::finalCh9DialogKey
```

### Additional context

_No response_