Add a new key pair object type: MBEDTLS_PK_OPAQUE, intended for
implementations of asymmetric cryptography operations that call an
external cryptographic module.
External cryptographic module engines must implement the API described
by a mbedtls_pk_info_t structure and, usually, a custom setup function.
Document the fields of the mbedtls_pk_info_t structure and the
requirements on a PK engine. Also document non-obvious aspects of the
behavior of the pk interface functions on opaque keys.
Change the interface of check_pair_func to take a pointer to a full
mbedtls_pk_context as its pub argument, and not just the data part of
the context. This is necessary because when prv is opaque, pub may
legitimately be of a different type (typically prv would be opaque and
pub would be transparent).
02768b436b