feat: Add support for custom volumes and volume mounts in HA chart

[as51340]

What

This PR adds support for custom volumes and volume mounts in the memgraph-high-availability chart for both data instances and coordinators. New values
storage.data.extraVolumes, storage.data.extraVolumeMounts, storage.coordinators.extraVolumes, and storage.coordinators.extraVolumeMounts are introduced in values.yaml.
The templates/data.yaml and templates/coordinators.yaml StatefulSet templates are updated to conditionally render extra volumes and include a hardcoded bolt-certs SSL
volume mount.

Why

Users need a way to mount arbitrary volumes (e.g. TLS certificates from Kubernetes Secrets) into Memgraph pods. The primary use case is mounting Bolt SSL certificates for
encrypted client connections, but the extraVolumes/extraVolumeMounts pattern is generic enough to support other scenarios (config files, additional storage, etc.).

How

• values.yaml: Added extraVolumes: [] and extraVolumeMounts: [] under both storage.data and storage.coordinators, with commented examples showing how to mount a TLS
  Secret as a volume.
• templates/data.yaml: Added a conditional volumes: block that renders storage.data.extraVolumes when non-empty. Added a hardcoded bolt-certs volume mount at
  /etc/memgraph/ssl (readOnly).
• templates/coordinators.yaml: Same changes as data.yaml but scoped to coordinator StatefulSets, using storage.coordinators.extraVolumes.