Add a protocol handler for AES CBC decryption with PKCS7 padding

This can later be extended to support other AES bit sizes,
encryption, other crypto algorithms, reading the key from a URL, etc.

In order to use it, the key and initialization vector has to be
passed via AVOptions. Since such options can't be passed to
protocols from the command line, the protocol is currently
only for libavformat internal use.

Signed-off-by: Martin Storsjö <martin@martin.st>