Update our EVP_PKEY_METHODs to get low level keys via public APIs

Update our EVP_PKEY_METHODs to get low level keys via public APIs

It is possible to call built-in EVP_PKEY_METHOD functions with a provided
key. For example this might occur if a custom EVP_PKEY_METHOD is in use
that wraps a built-in EVP_PKEY_METHOD. Therefore our EVP_PKEY_METHOD
functions should not assume that we are using a legacy key. Instead we
get the low level key using EVP_PKEY_get0_RSA() or other similar functions.
This "does the right thing" if the key is actually provided.

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/16118)