Allow setting PATH variable for each user dynamically

I have a conda environment per user in their home directory. so PATH needs to be something like `PATH=/home/{username}/conda/bin:$PATH`. However, currently there seems to be no way to do this, since username expansion isn't supported in environment files - plus PATH doesn't seem to be passed through to the spawned notebook (on Slurm at least)

### Proposed change

1. Allow username expansion via `{username}` or similar in PATH
2. Add a separate traitlet just for PATH, [similar to systemdspawner](https://github.com/jupyterhub/systemdspawner#extra_paths)

### Alternative options

Currently, I use the following to get what I want:

```python
class CustomSlurmSpawner(SlurmSpawner):
def start(self, *args, **kwargs):
self.req_prologue = f'export PATH=/home/{self.user.name}/conda/bin:$PATH'
return super().start(*args, **kwargs)

c.JupyterHub.spawner_class = CustomSlurmSpawner
```

### Who would use this feature?

People who have per-user conda environments