feat: added support for WAL-G SSH_PASSWORD

[kevinvalk]

The original omission of SSH_PASSWORD in #549 was motivated by general SSH best practices, which primarily apply to interactive, internet-exposed SSH usage. In the Spilo + WAL-G context, this assumption does not necessarily hold.

In Kubernetes-based, GitOps-managed deployments, both SSH identities and SSH passwords are ultimately stored as Kubernetes Secrets and therefore have the same protection level inside the cluster (RBAC, etcd encryption at rest, namespace isolation). From that perspective, there is no inherent security difference between the two once they are mounted into a pod.

Moreover, in many GitOps setups, SSH keys are generated outside the cluster and stored (encrypted) in Git, then decrypted by Flux or CI at deploy time. In contrast, an SSH_PASSWORD can be generated directly inside the cluster using standard secret generators, never leaving the cluster boundary at all. For those environments, this can reduce secret sprawl and external handling of long-lived credentials, which is a concrete and practical security improvement.

This PR does not argue that password-based SSH is universally preferable. It simply acknowledges that:

• Spilo is used in tightly controlled, often cluster-local backup topologies
• operational constraints differ between environments
• users should be able to choose the authentication mechanism that best fits their security model

When used as intended (cluster-local SSH, restricted network access, non-root users), enabling SSH_PASSWORD does not meaningfully weaken the security model, while significantly improving deployment flexibility and GitOps ergonomics.

For environments where SSH is exposed outside the cluster, SSH key or certificate-based authentication remains the recommended option.

closes #788 and #819

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