Metadata API: set default version for MetaFile()

This makes sense to me: if you create a new MetaFile, logically it
is version 1). This does not change serialization in any way.

Practical code becomes slightly nicer as
    metafiles = defaultdict(MetaFile)
now works without lambdas.

Signed-off-by: Jussi Kukkonen <jkukkonen@google.com>

Author: jku

tuf/api/metadata.py

@@ -1076,7 +1076,7 @@ class MetaFile(BaseFile):
 
     def __init__(
         self,
-        version: int,
+        version: int = 1,
         length: Optional[int] = None,
         hashes: Optional[Dict[str, str]] = None,
         unrecognized_fields: Optional[Dict[str, Any]] = None,