feat: optional existence checking

[cristobalgvera]

Added a new attribute called checkExistence that allows users to decide whether they want the existence check process for topics and subscriptions when creating the GCP Pub/Sub client instances.

This is closely related to the recent inclusion of the init parameter.

This change will helps users to accomplish two actions:

1. Avoid requiring additional GCP service account permissions to check existence.
2. Improve the speed when launching their projects due to the lack of existence checking.