[crypto] Use private data field for public-key algorithms

[crypto] Use private data field for public-key algorithms

Following the example of commit 25072c1 ("[crypto] Use private data
field for key exchange algorithms"), extend the definition of a
public-key algorithm to include an opaque private data field, and use
this to eliminate the wrapper functions for PKCS#1 and RSA-PSS.

Signed-off-by: Michael Brown <mcb30@ipxe.org>