Call kms_message to generate requests for encrypt and decrypt
Encrypt - https://cloud.google.com/kms/docs/reference/rest/v1/projects.locations.keyRings.cryptoKeys/encrypt
Decrypt - https://cloud.google.com/kms/docs/reference/rest/v1/projects.locations.keyRings.cryptoKeys/decrypt
Add mock testing equivalent to https://github.com/mongodb/mongo/blob/master/jstests/client_encrypt/fle_keys.js
jstests/fle/fle_implicit_encryption.js
Example: https://github.com/mongodb/libmongocrypt/blob/master/kms-message/test/test_ks_gcp_online.c