Just let me specify `imagePullSecret`

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

In our k8s cluster, we use a private registry. There's already a secret in that cluster, let's call it `the-pull-secret`, with the pull secret.

All of the pre-existing pods use that secret; now we'd like to add Cockroach to that cluster. Unlike most other charts out there, which just permit specifying an `imagePullSecret` in their `values.yaml`, Cockroach instead does this:

```
imagePullSecrets:
{{- if .Values.image.credentials }}
- name: {{ template "cockroachdb.fullname" . }}.db.registry
{{- end }}
```

So … as written, this means I cannot use the secret I already have; its name will never match.

Worse … setting `image.credentials` means that the Helm chart now generates a `Secret` object. Many of us (myself included) are doing some form of IaC: we essentially commit Helm inputs (e.g., ArgoCD), and sometimes Helm outputs (e.g., Flux) to Git. This makes it really clear what's changing in k8s's state, and offers trivial rollbacks of bad deployments.

However, it means that there cannot be secrets in Helm `values.yaml` files, or in the output of the template. (It's fine if it's _optional_; as long as I have a means to not put it there.)

**Describe the solution you'd like**
We already have means to get secrets, such as the pull secret, securely to the cluster. (E.g., we use `sealed-secrets`.) I just need the chart to allow me to say "this secret, exactly" for the pull secret, and to not attempt to generate that secret.

**Describe alternatives you've considered**
N/A

**Additional context**
See also [ArgoCD](https://argo-cd.readthedocs.io/en/stable/), [Flux](https://github.com/fluxcd/flux), [sealed-secrets](https://github.com/bitnami-labs/sealed-secrets)

Jira issue: CRDB-36651

Jira issue: HELM-45