tools/osbuild-mpp: hash mpp-embed urls from stream

Currently if one uses `mpp-embed` with URLs, osbuild-mpp still wants to
download the full file just so it can hash it. Make this more efficient
by hashing from the stream instead, which `hashlib` natively supports.

This also makes osbuild-mpp work with large artifacts in environments
that may not have enough space to temporarily save the data.

tools/osbuild-mpp

@@ -1608,20 +1608,23 @@ class ManifestFileV2(ManifestFile):
                 f, _ = self.find_and_open_file(path, [], mode="rb", encoding=None)
                 with f:
                     data = f.read()
+                checksum = hashlib.sha256(data).hexdigest()
             elif url:
                 response = urllib.request.urlopen(url)
-                data = response.fp.read()
+                h = hashlib.file_digest(response.fp, 'sha256')
+                checksum = h.hexdigest()
             else:
                 data = bytes(text, "utf-8")
+                checksum = hashlib.sha256(data).hexdigest()
 
-            checksum = hashlib.sha256(data).hexdigest()
             digest = "sha256:" + checksum
 
             if url:
                 source = element_enter(self.sources, "org.osbuild.curl", {})
                 items = element_enter(source, "items", {})
                 items[digest] = url
             else:
+                assert data is not None
                 encoded = base64.b64encode(data).decode("utf-8")
                 source = element_enter(self.sources, "org.osbuild.inline", {})
                 items = element_enter(source, "items", {})