make `hook` to accept array of values

### Proposal
https://developer.hashicorp.com/nomad/docs/job-specification/lifecycle#lifecycle-parameters
Allow `hook` to accept array

### Use-cases
We have cleanup task. The definition is the exactly the same for `prestart` and `poststop` hooks. It would be nice to configure them as:
```tf
task "cleanup-smthg" {
lifecycle {
hook = ["prestart", "poststop" ]
sidecar = false
}
...
}
```

### Attempted Solutions
Copy/paste hook and change its type: prestart -> poststop

Start with the lifecycle parameters documentation linked in the issue and locate the implementation and validation for lifecycle.hook. Trace how prestart and poststop are represented, then determine what tests cover hook values; done means one task definition can use both hooks in an array without breaking existing single-value configurations.

Written by the indexing model from the issue text.