Add open-ended trust policy for testing

Author: sarahchen6

.github/chainguard/self.pin-system-tests.create-pr.sts.yaml

@@ -0,0 +1,12 @@
+issuer: https://token.actions.githubusercontent.com
+
+subject_pattern: repo:DataDog/dd-trace-java:ref:refs/heads/*
+
+claim_pattern:
+  event_name: (push|workflow_dispatch)
+  ref: refs/heads/*
+  job_workflow_ref: DataDog/dd-trace-java/\.github/workflows/pin-system-tests\.yaml@refs/heads/master
+
+permissions:
+  contents: write
+  pull_requests: write