Add AES GCM to the SRT API.

[maxsharabayko]

API Changes

• Socket option SRTO_CRYPTOMODE where 0 means the standard AES-CTR; 1 enables AES GCM.
• SRT_KM_S_BADCRYPTOMODE to signal rejection due to crypto mode mismatch.
• New rejection reason SRT_REJ_CRYPTO.

Functional Changes

• Encrypt/decrypt using the AES GCM mode.
• When TSBPD is disabled, exclude the timestamp field from the encryption (Receiver, Sender).
• Exclude the retransmission flag from decryption (Receiver).
• Maybe temporarily disable AEAD if TSBPD is disabled.
• Break the connection if AEAD decryption has failed.

Documentation Updates

• Socket option SRTO_CRYPTOMODE.
• SRT_KM_S_BADCRYPTOMODE rejection reason.

Sample Applications

• Add URI query option cryptomode.

Unit Tests

• Test AES GCM via the CCryptoControl.

To Consider

• Maybe SRTO_CIPHERSUITE instead of SRTO_CRYPTOMODE?