Allow setting storage volumes through the TF module

### Enhancement Proposal

COS already exposes storage directive vars for worker and coordinator; issue is that it will apply indiscriminately to [all workers regardless](https://github.com/canonical/tempo-operators/blob/main/terraform/main.tf#L82) of the role. ideally we want to be able to tune storage directives on a per-role basis; however we can't really do that at the moment without breaking BC.

We could add one variable on top of the existing one, that will only apply to the worker with the WAL (`ingester`). TODO; double-check which workers have a WAL.

give it a variable that's future-proof: someday we'll be able to make this breaking change and we'll expose each worker's storage directives as a separate tf var. So give it a name that will make sense in that context; such as `pyroscope-worker-ingester-storage-directives` or something like that.

Set default to 50G.