Operator does not allow specification of pod security context to backup job pods

A user may wish to configure their Couchbase Server pods to run as a specific user using a security context, which is how you also define the fsgroup.

We allow users to specify this as part of https://docs.couchbase.com/operator/2.0/reference-couchbasecluster.html#spec-securitycontext which is passed directly down to the underlying Couchbase Server pods when creating in Kubernetes.
There is not a way to specify the same security context for the backup job pods, as we only extract the fsgroup - https://github.com/couchbase/couchbase-operator/blob/9f6a53fa2c83ebc9669168fd1692ec479be0b89a/pkg/cluster/backup.go#L187-L189.

This prevents deployment of the backup jobs in environments where runAsNonRoot is enforced (most secure environments) through pod security policies (https://kubernetes.io/docs/concepts/policy/pod-security-policy/#users-and-groups), due to the following error:

Error: container has runAsNonRoot and image has non-numeric user (couchbase), cannot verify user is non-root

Being able to specify the UID of 1000 in the pod security context for the backup would fix that issue.