JVET-AJ0151: Digitally Signed Content SEI messages

Ported from JVET-TuC with small improvements on certificate handling

Implements functionality to sign bitstreams to the encoder and verify bitstreams at the decoder.

If Digitally Signed Content SEI messages are present in the bitstream, the decoder prints the verification status.

Requires OpenSSL Version 3 for functionality beyond parsing (auto-detected).

Example command lines (assuming start from a subfolder eg. "run" in the main directory):

../bin/EncoderApp -c ../cfg/encoder_randomaccess_vtm.cfg -c ../cfg/per-sequence/RaceHorses.cfg  -c ../cfg/sei_vui/digitally_signed_content.cfg -ipp <sequence_directory> -f 33 --SEIDecodedPictureHash=1

../bin/DecoderApp -b str.bin --TrustStoreDir=../cfg/keystore/ca --KeyStoreDir=../cfg/keystore/public/