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+WKXuxeK9yCbC9jqEchwjCNsQ4RXAsJ07oHZ+Q
+qz55cNIY5BAhcQ0QYY5jv9BimGL9SuhUuOSuS3Pn9rrsIFGcFsihy4kDr8rq5+7RbN8bV057gW5
+emYjLtvDsQ
+-----END PUBLIC KEY-----
+
+$ openssl asn1parse -i < [PUBLIC KEY]
+    0:d=0  hl=2 l= 118 cons: SEQUENCE          
+    2:d=1  hl=2 l=  16 cons:  SEQUENCE          
+    4:d=2  hl=2 l=   7 prim:   OBJECT            :id-ecPublicKey
+   13:d=2  hl=2 l=   5 prim:   OBJECT            :secp384r1
+   20:d=1  hl=2 l=  98 prim:  BIT STRING        
+
+
+
+-----BEGIN ALGORITHM-----
+MAoGCCqGSM49BAMC
+-----END ALGORITHM-----
+
+$ openssl asn1parse -i < [ALGORITHM]
+    0:d=0  hl=2 l=  10 cons: SEQUENCE          
+    2:d=1  hl=2 l=   8 prim:  OBJECT            :ecdsa-with-SHA256
+
+
+
+-----BEGIN DATA-----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+-----END DATA-----
+
+
+
+-----BEGIN SIGNATURE-----
+A2kAMGYCMQCDwMSZVJZ8qThiNTV7VX57m8fdNnNS6cXIjRYoh4SozlYWmWGh87nhmJJCeD16jVM
+CMQDkroAY8oNi8RxLUor+LozhhVgu24tu6lcp4MP8H3Zy8bBea5HerAd1AqJp3Ox7KDU=
+-----END SIGNATURE-----
+
+$ openssl asn1parse -i < [SIGNATURE]
+    0:d=0  hl=2 l= 105 prim: BIT STRING