feat(gcp): add Pub/Sub subscription management

[atharva-nagane]

Closes #104

Summary

• Adds gcp_pubsub_subscription.rs — the consumer-side complement to the existing
  gcp_notification_service.rs which only handles topics
• 6 operations covering the full subscription lifecycle + message flow
• No new dependencies; uses existing retrieve_token() auth helper

Operations

Method	Endpoint
create_subscription	PUT /v1/projects/{p}/subscriptions/{s}
delete_subscription	DELETE /v1/projects/{p}/subscriptions/{s}
list_subscriptions	GET /v1/projects/{p}/subscriptions
pull_messages	POST /v1/projects/{p}/subscriptions/{s}:pull
acknowledge_messages	POST /v1/projects/{p}/subscriptions/{s}:acknowledge
modify_push_config	POST /v1/projects/{p}/subscriptions/{s}:modifyPushConfig

Test plan

• cargo build passes with no new dependencies
• cargo test gcp_pubsub runs without compile errors
• Integration tests require GOOGLE_APPLICATION_CREDENTIALS set to a valid service account