chore: add pubsub and bigtable downstream dependency checks (#397)

Author: Neenu1995

.github/workflows/downstream.yaml

@@ -16,6 +16,7 @@ jobs:
  - java-bigqueryconnection
  - java-spanner
  - java-storage
+ - java-pubsub
  steps:
  - uses: actions/checkout@v2
  - uses: actions/setup-java@v1