Implement VerifySignedData() for ECDSA, RSA PKCS#1 and RSA PSS.

net/data/verify_signed_data_unittest/ecdsa-secp384r1-sha256.pem

+This test data was produced by creating a self-signed EC cert using OpenSSL,
+and then extracting the relevant fields.
+
+It uses ECDSA with curve secp384r1 an SHA-256.
+
+(1) Generate self-signed certificate
+
+  openssl ecparam -out ec_key.pem -name secp384r1 -genkey 
+  openssl req -new -key ec_key.pem -x509 -nodes -days 365 -out cert.pem
+
+(2) Extract public key
+
+  openssl x509 -in cert.pem -pubkey -noout > pubkey.pem
+  cat pubkey.pem
+
+(3) Extract signed data (tbsCertificate)
+
+  openssl asn1parse -in cert.pem -out tbs -noout -strparse 4
+  base64 tbs
+
+(4) Extract signature algorithm
+
+  # Find the offset of the signature algorithm near the end (496 in this case)
+  openssl asn1parse -in cert.pem
+
+  openssl asn1parse -in cert.pem -out alg -noout -strparse 496
+  base64 alg
+
+(5) Extract the signature
+
+  # Find the final offset of BIT STRING (508 in this case)
+  openssl asn1parse -in cert.pem
+
+  openssl asn1parse -in cert.pem -out sig -noout -strparse 508
+  base64 sig
+
+
+-----BEGIN PUBLIC KEY-----
+MHYwEAYHKoZIzj0CAQYFK4EEACIDYgAEXFhBi+WKXuxeK9yCbC9jqEchwjCNsQ4R
+XAsJ07oHZ+Qqz55cNIY5BAhcQ0QYY5jv9BimGL9SuhUuOSuS3Pn9rrsIFGcFsihy
+4kDr8rq5+7RbN8bV057gW5emYjLtvDsQ
+-----END PUBLIC KEY-----
+
+-----BEGIN ALGORITHM-----
+MAoGCCqGSM49BAMC
+-----END ALGORITHM-----
+
+-----BEGIN DATA-----
+MIIB6KADAgECAgkAtUVxft6/Vc0wCgYIKoZIzj0EAwIwbzELMAkGA1UEBhMCQVUxEzARBgNVBAgM
+ClNvbWUtU3RhdGUxITAfBgNVBAoMGEludGVybmV0IFdpZGdpdHMgUHR5IEx0ZDEMMAoGA1UEAwwD
+YWhhMRowGAYJKoZIhvcNAQkBFgtzdXBAZm9vLmNvbTAeFw0xNTA3MDIwMDM4MTRaFw0xNjA3MDEw
+MDM4MTRaMG8xCzAJBgNVBAYTAkFVMRMwEQYDVQQIDApTb21lLVN0YXRlMSEwHwYDVQQKDBhJbnRl
+cm5ldCBXaWRnaXRzIFB0eSBMdGQxDDAKBgNVBAMMA2FoYTEaMBgGCSqGSIb3DQEJARYLc3VwQGZv
+by5jb20wdjAQBgcqhkjOPQIBBgUrgQQAIgNiAARcWEGL5Ype7F4r3IJsL2OoRyHCMI2xDhFcCwnT
+ugdn5CrPnlw0hjkECFxDRBhjmO/0GKYYv1K6FS45K5Lc+f2uuwgUZwWyKHLiQOvyurn7tFs3xtXT
+nuBbl6ZiMu28OxCjUDBOMB0GA1UdDgQWBBR6nDgtPalG2JvSlWzdGRCi/Cu7NjAfBgNVHSMEGDAW
+gBR6nDgtPalG2JvSlWzdGRCi/Cu7NjAMBgNVHRMEBTADAQH/
+-----END DATA-----
+
+-----BEGIN SIGNATURE-----
+MGYCMQCDwMSZVJZ8qThiNTV7VX57m8fdNnNS6cXIjRYoh4SozlYWmWGh87nhmJJCeD16jVMCMQDk
+roAY8oNi8RxLUor+LozhhVgu24tu6lcp4MP8H3Zy8bBea5HerAd1AqJp3Ox7KDU=
+-----END SIGNATURE-----