This commit introduces new options to support more than one decryption
keys:
* add a decryption_keys option to MOV, that supports a dictionary of
KID=>key (in hex), using AV_OPT_TYPE_DICT
* add the corresponding cenc_decryption_keys option to DASH
Signed-off-by: Kacper Michajłow <kasper93@gmail.com>