Add helper function to parse signing algorithm according to rfc

Altough it is an option, it may not be beneficial to directly adjust the
enum to mirror the standard described in the rfc because
(1) existing cosign signatures would not be reused but instead new
signatures would be appended because it seems the algorithm had changed
(2) OCM cli expects algorithms to be upper case

model/signing_server.py

@@ -15,6 +15,18 @@ class SigningAlgorithm(enum.StrEnum):
     RSASSA_PSS = 'rsassa-pss'
     RSASSA_PKCS1_V1_5 = 'rsassa-pkcs1-v1_5'
 
+    @staticmethod
+    def as_rfc_standard(algorithm: 'SigningAlgorithm' | str) -> str:
+        # parses the algorithm to the standard format described in
+        # https://datatracker.ietf.org/doc/html/rfc3447
+        algorithm = SigningAlgorithm(algorithm.lower())
+        if algorithm is SigningAlgorithm.RSASSA_PSS:
+            return 'RSASSA-PSS'
+        elif algorithm is SigningAlgorithm.RSASSA_PKCS1_V1_5:
+            return 'RSASSA-PKCS1-v1_5'
+        else:
+            raise NotImplementedError(algorithm)
+
 
 class SigningServerEndpoint(NamedModelElement):
     def url(self) -> str: