This is in preparation to adding support for opaque keys in the pk layer. The
header files are for use as follows:
* pk.h for users of the pk interface.
* pk_info.h for implementers of opaque key engines.
* pk_internal.h only inside libmbedcrypto to implement built-in pk types.
Expose a function mbedtls_pk_signature_size to calculate the maximum
size of a signature made with a given key. Document that this is the
buffer size that mbedtls_pk_sign requires.
Add a corresponding field signature_size_func to the mbedtls_pk_info
structure.
In the mbedtls_pk_info_t method can_do, pass the context data. This
will be needed for opaque keys, where the info structure depends on
the method to access the opaque key and not on the key type.
