Support for Google Cloud

Add support for Google Secret Manager (GSM) as a secret backend in consul-template, similar to existing integrations with Vault and AWS Secrets Manager.

**Use Case**
Currently, consul-template supports fetching secrets dynamically from HashiCorp Vault and AWS Secrets Manager. However, many users rely on Google Cloud Secret Manager (GSM) to store application secrets in GCP environments.

Integrating GSM would allow consul-template to natively fetch and inject secrets into configuration files, reducing the need for workarounds such as:

- Using an initContainer with gcloud CLI to retrieve secrets.
- Manually syncing secrets from GSM to Vault.
- Writing custom scripts to fetch secrets before running consul-template.

Start by reading the existing Vault and AWS Secrets Manager integrations to understand how secret backends are represented and consumed. Then determine the Google Secret Manager API and authentication requirements, and define the tests needed to show that GSM secrets can be fetched and injected into rendered configuration files.

Written by the indexing model from the issue text.