chore(gcp_pubsub): add stricter compile/xref flags

apps/emqx_bridge_gcp_pubsub/rebar.config

@@ -1,10 +1,26 @@
 %% -*- mode: erlang; -*-
-{erl_opts, [debug_info]}.
+{erl_opts, [
+    warn_unused_vars,
+    warn_shadow_vars,
+    warn_unused_import,
+    warn_obsolete_guard,
+    warnings_as_errors,
+    debug_info
+]}.
 {deps, [ {emqx_connector, {path, "../../apps/emqx_connector"}}
        , {emqx_resource, {path, "../../apps/emqx_resource"}}
        , {emqx_bridge, {path, "../../apps/emqx_bridge"}}
        ]}.
 
+{xref_checks, [
+    undefined_function_calls,
+    undefined_functions,
+    locals_not_used,
+    deprecated_function_calls,
+    warnings_as_errors,
+    deprecated_functions
+]}.
+
 {shell, [
     {apps, [emqx_bridge_gcp_pubsub]}
 ]}.