Rename Envelope sign and verify methods

sign is renamed to create_sig and verify is renamed to verify_sigs,
so it does not clashes with other methods of current signature
wrappers.

This has to be done to avoid any breaking change while integrating
DSSE in in-toto and tuf projects.

Signed-off-by: Pradyumna Krishna <git@onpy.in>

securesystemslib/metadata.py

@@ -111,7 +111,7 @@ def pae(self) -> bytes:
             self.payload,
         )
 
-    def sign(self, signer: Signer) -> Signature:
+    def create_sig(self, signer: Signer) -> Signature:
         """Sign the payload and create the signature.
 
         Arguments:
@@ -126,7 +126,7 @@ def sign(self, signer: Signer) -> Signature:
 
         return signature
 
-    def verify(self, keys: List[Key], threshold: int) -> Dict[str, Key]:
+    def verify_sigs(self, keys: List[Key], threshold: int) -> Dict[str, Key]:
         """Verify the payload with the provided Keys.
 
         Arguments: