Implement key committing AEAD or AES-CTR with HMAC-SHA512 in python using cryptography pypi module/package

[Anish-M-code]

Currently none of AEAD ciphers in cryptography package support key commitment , but key commitment is a good to have property
refer discussions on https://crypto.stackexchange.com/questions/88716/understanding-the-impact-of-partitioning-oracle-attacks-on-stream-ciphers and https://crypto.stackexchange.com/questions/6075/is-encrypthmac-stronger-than-aead?noredirect=1&lq=1

I strongly believe its worthwhile to implement a key committing AEAD like AES-CTR with HMAC-SHA512 ,its unlikely to suffer major crypto mistakes if coded properly, any future contributors may refer discussion here https://crypto.stackexchange.com/questions/101612/common-pitfalls-to-be-taken-care-of-while-implementing-encrypt-then-hmac-scheme and guidelines here https://github.com/samuel-lucas6/Cryptography-Guidelines to replace AES-GCM used in PQcrypt.