add missing signer/verifier to DSAPublicKey and DSAPrivateKey ifaces
add missing signer/verifier to DSAPublicKey and DSAPrivateKey ifaces

These were present in the docs and the OpenSSL provider, but not
properly listed on the interface

Hasimir/cryptography,Ayrx/cryptography,kimvais/cryptography,sholsapp/cryptography,Ayrx/cryptography,skeuomorf/cryptography,dstufft/cryptography,dstufft/cryptography,skeuomorf/cryptography,bwhmather/cryptography,kimvais/cryptography,kimvais/cryptography,kimvais/cryptography,dstufft/cryptography,bwhmather/cryptography,Ayrx/cryptography,bwhmather/cryptography,dstufft/cryptography,sholsapp/cryptography,Hasimir/cryptography,Hasimir/cryptography,skeuomorf/cryptography,Ayrx/cryptography,bwhmather/cryptography,sholsapp/cryptography,skeuomorf/cryptography,sholsapp/cryptography,dstufft/cryptography,Hasimir/cryptography