Add backend bucket signed URL key (for CDN) support (#209)

Signed-off-by: Modular Magician <magic-modules@google.com>

plugins/modules/gcp_compute_backend_service_facts.py

@@ -206,6 +206,17 @@ items:
               - "'&' and '=' will be percent encoded and not treated as delimiters."
               returned: success
               type: list
+        signedUrlCacheMaxAgeSec:
+          description:
+          - Maximum number of seconds the response to a signed URL request will be
+            considered fresh, defaults to 1hr (3600s). After this time period, the
+            response will be revalidated before being served.
+          - 'When serving responses to signed URL requests, Cloud CDN will internally
+            behave as though all responses from this backend had a "Cache-Control:
+            public, max-age=[TTL]" header, regardless of any existing Cache-Control
+            header. The actual headers served in responses will not be altered.'
+          returned: success
+          type: int
     connectionDraining:
       description:
       - Settings for connection draining.