[JENKINS-72444] Add support for generic ephemeral storage

[idlefella]

Implemented support for generic ephemeral volumes: https://kubernetes.io/docs/concepts/storage/ephemeral-volumes/#generic-ephemeral-volumes

The PR adds the option "generic ephemeral volume" for volumes and workspace volumes. This has about the same behaviour as using a dynamic PVC, but uses the native kubernetes feature for that.

The code is unit tested and there are tests for CasC.

Resolves: https://issues.jenkins.io/browse/JENKINS-72444

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[idlefella]

Hi, the tests didn't pass due to a network error:

Error response from daemon: Head "https://registry-1.docker.io/v2/library/busybox/manifests/latest": read tcp 10.200.2.11:58444->34.226.69.105:443: read: connection reset by peer

Can you please retry them?