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

net/data/verify_signed_data_unittest/rsa-pkcs1-sha256.pem

+This test data was produced by creating a self-signed EC cert using OpenSSL,
+and then extracting the relevant fields.
+
+It uses RSA PKCS#1 v1.5 with SHA-256 and a 1024-bit key.
+
+(1) Generate self-signed certificate
+
+  openssl genrsa -out rsa_key.pem 1024
+  openssl req -new -key rsa_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 (491 in this case)
+  openssl asn1parse -in cert.pem
+
+  openssl asn1parse -in cert.pem -out alg -noout -strparse 491
+  base64 alg
+
+(5) Extract the signature
+
+  # Find the final offset of BIT STRING (506 in this case)
+  openssl asn1parse -in cert.pem
+
+  openssl asn1parse -in cert.pem -out sig -noout -strparse 506
+  base64 sig
+
+
+-----BEGIN PUBLIC KEY-----
+MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQCqR+B2Mj1irNp1JnY2zQIQRQPz
+7ybs6mUjHT3hf5APyaig2u6fBPThqxssgg0YviqIj/70hVK/JXcqP1zgR5AhsA0a
+gnwAjfB/ow4EH+3HEYV52qpxN98pUxC+1l2hgVtn8xCS/JGyjK+6dv+lZw3ixJoX
+2RjVtdJ4/cn318EhhQIDAQAB
+-----END PUBLIC KEY-----
+
+-----BEGIN ALGORITHM-----
+MA0GCSqGSIb3DQEBCwUA
+-----END ALGORITHM-----
+
+-----BEGIN DATA-----
+MIIB46ADAgECAgkA3l4tFOVii0UwDQYJKoZIhvcNAQELBQAwVjELMAkGA1UEBhMCQVUxEzARBgNV
+BAgMClNvbWUtU3RhdGUxITAfBgNVBAoMGEludGVybmV0IFdpZGdpdHMgUHR5IEx0ZDEPMA0GA1UE
+AwwGTXkga2V5MB4XDTE1MDcwMjE3MDYzOVoXDTE2MDcwMTE3MDYzOVowVjELMAkGA1UEBhMCQVUx
+EzARBgNVBAgMClNvbWUtU3RhdGUxITAfBgNVBAoMGEludGVybmV0IFdpZGdpdHMgUHR5IEx0ZDEP
+MA0GA1UEAwwGTXkga2V5MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQCqR+B2Mj1irNp1JnY2
+zQIQRQPz7ybs6mUjHT3hf5APyaig2u6fBPThqxssgg0YviqIj/70hVK/JXcqP1zgR5AhsA0agnwA
+jfB/ow4EH+3HEYV52qpxN98pUxC+1l2hgVtn8xCS/JGyjK+6dv+lZw3ixJoX2RjVtdJ4/cn318Eh
+hQIDAQABo1AwTjAdBgNVHQ4EFgQUzQBVKTEknyLndWd2HTsBdTKvyikwHwYDVR0jBBgwFoAUzQBV
+KTEknyLndWd2HTsBdTKvyikwDAYDVR0TBAUwAwEB/w==
+-----END DATA-----
+
+-----BEGIN SIGNATURE-----
+OsdKYVInD8a/uGzk83OlonACuL+yK1qshl/M39KNH6fwvzJ5MSiINvX4GTGUW9e88dNo4qg7daWy
+U007GHVbn8mRhgY5SQJzIiOeWfqpLnrSKZyvdU2OiZP8zwzYiyz6smIpq7VNG5o/sfyLr4ap/exo
+Z2vz8ue8UB01C+RqVC0=
+-----END SIGNATURE-----