feat: Add option to add topic publishers

[omer2500]

Use Case:

Currently, this module only allows default Google service accounts to publish messages to a Pub/Sub topic. However, there are many scenarios where additional members — such as service accounts used by microservices — need publish permissions for specific topics.

Proposed Solution:

Introduce a new variable (similar to bucket_creators in the Cloud Storage module) that accepts a list of IAM members. These members will be granted the pubsub.publisher role on the topic, allowing for more flexible and explicit control over who can publish messages.

For example:

topic_publishers = [
   "serviceAccount:[email protected]",
   "group:[email protected]"
]

[omer2500]

Hi,

@ayushmjain @imrannayer @q2w

any update on this:)? would be great to get review