The new API is more generic to the KMS providers. Instead of KMS provider specific API, the GCP and Azure KMS providers can both be set through:

bool
mongocrypt_setopt_kms_providers (mongocrypt_t *crypt, mongocrypt_binary_t *kms_providers);

kms_providers is a BSON document which accepts keys for "gcp" and "azure". It closely matches the spec, and means no additional API is necessary to support future fields or new KMS providers. Currently it only accepts "gcp" and "azure". It would simplify binding code to support "aws" and "local".

Similarly, the key encryption key (aka masterKey) set for a create datakey operation can be set through:

bool
mongocrypt_ctx_setopt_key_encryption_key (mongocrypt_ctx_t *ctx, mongocrypt_binary_t *bin);

bin includes the "provider" field. That also only supports "gcp" and "azure" currently.