protos/PublicKeyDetails: add compatibility algorithms using SHA256 (#572)

Signed-off-by: Riccardo Schirone <[email protected]>

Author: ret2libc

gen/jsonschema/schemas/ArtifactVerificationOptions.schema.json

@@ -85,6 +85,11 @@ enum PublicKeyDetails {
         PKIX_ED25519 = 7; // See RFC8032
         PKIX_ED25519_PH = 8;
 
+        // These algorithms are deprecated and should not be used, but they
+        // were/are being used by most Sigstore clients implementations.
+        PKIX_ECDSA_P384_SHA_256 = 19 [deprecated = true];
+        PKIX_ECDSA_P521_SHA_256 = 20 [deprecated = true];
+
         // LMS and LM-OTS
         //
         // These keys and signatures may be used by private Sigstore
@@ -102,7 +107,7 @@ enum PublicKeyDetails {
         LMOTS_SHA256 = 15;
 
         // Reserved for future additions of public key/signature algorithm types.
-        reserved 19 to 50;
+        reserved 21 to 50;
 }
 
 // HashOutput captures a digest of a 'message' (generic octet sequence)