Support TLS options for KMS provider connections

/**

 * Get the KMS provider identifier associated with this KMS request.

 *

 * This is used to conditionally configure TLS connections based on the KMS

 * request. It is useful for KMIP, which authenticates with a client

 * certificate.

 *

 * @param[in] kms The @ref mongocrypt_kms_ctx_t object.

 * @param[out] len Receives the length of the returned string.

 *

 * @returns One of the static strings: "aws", "azure", "gcp", or "kmip".

 */

MONGOCRYPT_EXPORT

const char *

mongocrypt_kms_ctx_get_kms_provider (mongocrypt_kms_ctx_t *kms, uint32_t *len);