Secrets: Add Proxy backend Signed-off-by: Simo Sorce <simo@redhat.com> Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>

Secrets: Add encryption at rest Generates a master key file if it doesn't exist and encrypts secrets using the master key contained in the file. Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>

Add initial providers infrastructure. Also adds support for the basic LOCAL provider that stores data on the local machine. Signed-off-by: Simo Sorce <simo@redhat.com> Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>