EVP_PKEY_assign_EC_KEY(): detect SM2 curve and set EVP_PKEY type accordingly

EVP_PKEY_assign_EC_KEY(): detect SM2 curve and set EVP_PKEY type accordingly

This means that when loaded or created, EC EVP_PKEYs with the SM2
curve will be regarded as EVP_PKEY_SM2 type keys by default.
Applications are no longer forced to check and fix this.

It's still possible, for those who want this, to set the key type to
EVP_PKEY_EC and thereby run the normal EC computations with the SM2
curve.  This has to be done explicitly.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/10942)