Add ucx to PMC's extended repo (#11831)

Author: ellie-di

SPECS-EXTENDED/ucx/ucx.signatures.json

@@ -0,0 +1,5 @@
+{
+ "Signatures": {
+  "ucx-1.18.0.tar.gz": "C5DDBF6ADE53415CC2402FA540281C92958B67D3B1DD2B4D721D4019616B88D2"
+ }
+}