scsi: target: iscsi: Support base64 in CHAP

scsi: target: iscsi: Support base64 in CHAP

RFC7143 allows both Base64 and Hex encoding for CHAP binary entities like
Challenge and Response. Currently the Linux iSCSI target supports only Hex
encoding.

Add support for Base64 encoded CHAP Challenge and CHAP Response required
for CHAP tests in Windows HLK.

Link: https://lore.kernel.org/r/20220718152555.17084-3-d.bogdanov@yadro.com
Reviewed-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Dmitry Bogdanov <d.bogdanov@yadro.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>