[google_cloud_storage] update gcs mock-service for system test

[brijesh-elastic]

Proposed commit message

google_cloud_storage: update gcs mock-service for system test

The previously used image (fsouza/fake-gcs-server:latest) for system tests is no longer 
compatible. It is currently being updated with a mock service that supports essential 
SDK API calls for the GCS input.

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
• I have verified that Kibana version constraints are current according to guidelines.
• I have verified that any added dashboard complies with Kibana's Dashboard good practices

How to test this PR locally

• Clone integrations repo.
• Install elastic package locally.
• Start elastic stack using elastic-package.
• Move to integrations/packages/google_cloud_storage directory.
• Run the following command to run tests.

elastic-package test

[elasticmachine]

Pinging @elastic/security-service-integrations (Team:Security-Service Integrations)

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 86ad2bb

cc @brijesh-elastic

[ShourieG]

LGTM, I will create an issue to update these input packages to support the latest config options so we have better support as many new features have been added since 8.13