Make the default auto-requeue signal in SLURM environments configurable

Nobody has claimed this yet.

### Description & Motivation

I have to use a requeue-signal other than `SIGUSR1` for technical reasons, which is the hard coded default at the moment. While I can use `SLURMEnvironment(requeue_signal=signal.SIGUSR2)` as a plugin, I would like to only set that in cases where I actually am in a SLURM environment without having to re-implement the environment detection inside of lightning. In other words, I still want to be able to just run my script locally in a `LightningEnvironment`.

### Pitch

I want to move the hard-coded constant [here](https://github.com/Lightning-AI/pytorch-lightning/blob/master/src/lightning/fabric/plugins/environments/slurm.py#L51) into a class constant, so that I can change it from the outside.

### Alternatives

Re-implementing environment detection myself so that I can pass an argument to just the SLURM environment, but that seems like too much work and needless duplication.

### Additional context

_No response_

cc @lantiga

Start with src/lightning/fabric/plugins/environments/slurm.py and inspect the hard-coded requeue signal and how SLURMEnvironment is selected. Make the default signal externally configurable while preserving automatic local LightningEnvironment use; done means callers can change the SLURM default without reimplementing environment detection.

Written by the indexing model from the issue text.