cmd: add ca-signed tool.

Verify certificates are signed by a CA.

cmd/ca-signed/testdata/goog-wr2.pem

@@ -0,0 +1,29 @@
+-----BEGIN CERTIFICATE-----
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+-----END CERTIFICATE-----

cmd/ca-signed/testdata/gts-r1.pem

@@ -0,0 +1,31 @@
+-----BEGIN CERTIFICATE-----
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+-----END CERTIFICATE-----

cmd/ca-signed/testdata/isrg-root-x1.pem

@@ -0,0 +1,31 @@
+-----BEGIN CERTIFICATE-----
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+-----END CERTIFICATE-----

cmd/ca-signed/testdata/le-e7.pem

@@ -0,0 +1,26 @@
+-----BEGIN CERTIFICATE-----
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+-----END CERTIFICATE-----