Add encryption support the k8s OpenStack `cinder-csi-plugin`

[NotTheEvilOne]

As a SCS Operator, I want to use OpenStack the cinder-csi-plugin with encrypted volumes so that I can use encryption at rest for sensitive data. Enhancements documented at #514 are required to add this support.

Definition of Ready:

• [x] User Story is clear and understood by the whole team
• [x] Acceptance criteria are defined
• [x] Acceptance criteria are clear and understood by the whole team

Acceptance criteria:

• [x] Changes are implemented in a branch
• [x] PR for basic encryption is created and linked
• [ ] PR for basic encryption is accepted
• [ ] PR for encryption based on customer keys is created and linked
• [ ] PR for encryption based on customer keys is accepted

Definition of Done:

• [ ] All acceptance criteria are met
• [ ] Changes have been reviewed
• [ ] Documentation has been updated
• [ ] Release Notes have been updated

[NotTheEvilOne]

Working with the actual code revealed missing support to predefine a user selected key for encryption. First basic implementation will focus on a encrypted parameter basically implementing validation and cleanup for the expected volume encryption therefore.

[NotTheEvilOne]

PR for the basic implementation is created https://github.com/kubernetes/cloud-provider-openstack/pull/2525

[NotTheEvilOne]

Blueprint and Spec for the "bring your own keys".