Adding Server-Generated Message ID To Received Message Attributes

[arthurspa]

This merge request resolves the issue I raised in #430 regarding missing server-generated message IDs in Watermill's Google Cloud Pub/Sub implementation. The change made is:

Server-Side Message ID Inclusion:
Modified the googlecloud package's default unmarshaller method to include Google's server-generated message IDs in message attributes, enabling users to access them.
A custom metadata key was created to avoid clash with existing implementations.

Test Coverage:
Added tests to ensure message IDs are present in received messages,

Expected Impact:
Improved compatibility with Google Cloud Pub/Sub.

Additional Notes:

Feedback is welcome for further refinement.

[arthurspa]

Hi @m110 , as we discussed in Discord, I've pushed a commit that sets the generated message ID to the message attributes after the message is published.

I wonder if should I call mutex.Lock() before setting the message attribute inside the Publish method though, to make it thread safe....

[m110]

Thank you @arthurspa!

I think the lock is not needed, as the mutex locks the publisher's internals, not the message being handled.