Certain tokens require data in the form of a `_mongocrypt_buffer_t` to be passed through upon initialization. As part of SERVER-96316, this is handled in the token refactor by using a `LibBSONValue` as an argument. However, `LibBSONValue` holds a `bson_value_t`, making it difficult to convert to `_mongocrypt_buffer_t` using current functions

A helper function `_mongocrypt_buffer_from_value` will make this process easier for the server team. This function will be very similar to `_mongocrypt_buffer_from_iter`