s390/uv: Retrieve UV secrets support

s390/uv: Retrieve UV secrets support

Provide a kernel API to retrieve secrets from the UV secret store.
Add two new functions:
* `uv_get_secret_metadata` - get metadata for a given secret identifier
* `uv_retrieve_secret` - get the secret value for the secret index

With those two functions one can extract the secret for a given secret
id, if the secret is retrievable.

Reviewed-by: Christoph Schlameuss <schlameuss@linux.ibm.com>
Signed-off-by: Steffen Eiden <seiden@linux.ibm.com>
Link: https://lore.kernel.org/r/20241024084107.2418186-1-seiden@linux.ibm.com
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>