filterx pubsub message

[bshifter]

Add pubsub_message() function to FilterX and extend Pub/Sub destination with protovar

This PR introduces new functionality for managing Google Pub/Sub messages in syslog-ng:

1. pubsub_message() Function:

• Adds a FilterX representation of the Google Pub/Sub message Protobuf descriptor.
• Enables users to define and manipulate Pub/Sub message content directly within the FilterX scripting module (source-side).

2. protovar Option in Pub/Sub Destination:

• Extends the Pub/Sub destination module with the protovar option.
• Allows referencing the FilterX-defined Pub/Sub Protobuf variable for setting the outgoing message payload.
  Together, these changes provide greater flexibility and control over Pub/Sub destination messages, supporting advanced customization directly from FilterX.

example:

filterx {
    # Adding data and attributes 
    $x = pubsub_message($MESSAGE, {"foo":{"bar":"baz"}});
};

log {
    destination {
        google-pubsub-grpc(
            ...
            proto_var($x)
        );
    };
};

[github-actions]

This Pull Request introduces config grammar changes

axoflow/e814ffbb404072b7e293fbfca97989c1796b11f9 -> bshifter/fx-pubsub

--- a/destination
+++ b/destination

 google-pubsub-grpc(
+    proto-var(
+        <template-content>
+        <template-reference>
+    )
 )

[alltilla]

Thanks for the updates on the PR. I found a couple more things that I unfortunately missed the last time.