aes: add AES encryption and decryption elements

This element adds support for AES encryption and decryption using the OpenSSL library.

Here are more details on AES encryption with OpenSSL.

Here is a sample pipeline to encrypt and then decrypt some text:

$ echo "0123456789ABCDEF012345678" > plain.txt
$ gst-launch-1.0 filesrc location=plain.txt ! aesenc key=1f9423681beb9a79215820f6bda73d0f \
iv=e9aa8e834d8d70b7e0d254ff670dd718  ! aesdec key=1f9423681beb9a79215820f6bda73d0f \
iv=e9aa8e834d8d70b7e0d254ff670dd718! filesink location=dec.txt
$ cat dec.txt