tuf.api: add helpers to bump version and expiration

Signed-off-by: Joshua Lock <jlock@vmware.com>

tuf/api/metadata.py

@@ -69,6 +69,12 @@ def signable(self) -> JsonDict:
         """
         raise NotImplementedError()
 
+    def bump_version(self) -> None:
+        self.version = self.version + 1
+
+    def bump_expiration(self, delta: relativedelta = relativedelta(days=1)) -> None:
+        self.expiration = self.expiration + delta
+
     def signed(self) -> str:
         return encode_canonical(self.signable['signed']).encode('utf-8')