Metadata API: Add simple threshold validation

Probably there could be future API calls that modify "threshold"
to a new value, but the problem is we don't have a clear idea
if they would exist and what exactly they will do.
That's why it makes sense to validate against the potential problems
we can imagine - in this case, is passing a threshold below 1.

Signed-off-by: Martin Vrachev <mvrachev@vmware.com>

tuf/api/metadata.py

@@ -506,6 +506,8 @@ def __init__(
                 f"keyids should be a list of unique strings,"
                 f" instead got {keyids}"
             )
+        if threshold < 1:
+            raise ValueError("threshold should be at least 1!")
         self.keyids = keyids_set
         self.threshold = threshold
         self.unrecognized_fields: Mapping[str, Any] = unrecognized_fields or {}