Allow defining the config with the backend in a `Secret` instead of a `ConfigMap`

#### Is your feature request related to a problem? Please describe.

Currently the suggested (and only way?) way to configure a [Pyroscope backend](https://grafana.com/docs/pyroscope/latest/configure-server/configure-object-storage-backend/) in [the Helm chart is by writing the credentials in plain text in the values](https://github.com/grafana/pyroscope/blob/854639b2d2c2945a9d8caed191ec537f1927da33/operations/pyroscope/helm/pyroscope/values.yaml#L156-L166).

#### Describe the solution you'd like

Allow defining the config with the backend in a `Secret` instead of a `ConfigMap`.

This could be done with a key in the `values.yaml` to switch between `ConfigMap` to defining a `Secret` to mount in the pods. Then the secret can be provisioned externally with each one's method.

#### Describe alternatives you've considered

Using the provider default logic. In my case [with GCP GCS](https://grafana.com/docs/pyroscope/latest/configure-server/reference-configuration-parameters/#gcs_storage_backend), with the JSON file where the environment variable `GOOGLE_APPLICATION_CREDENTIALS` defines or the well-known `$HOME/.config/gcloud/application_default_credentials.json`, or with the metadata server and the flag `-storage.gcs.service-account`.