Add lookup plugin for Google Secret Manager

##### SUMMARY
Add in a lookup plugin to retrieve secrets from [Google Secret Manager](https://cloud.google.com/secret-manager). [Similar to the one for AWS](https://docs.ansible.com/ansible/latest/collections/amazon/aws/aws_secret_lookup.html).

##### ISSUE TYPE
- Feature Idea

##### COMPONENT NAME

Maybe something like `google.cloud.gcp_secret`?

##### ADDITIONAL INFORMATION

A Lookup plugin would be a very convenient and more secure way to store passwords in Google Secret Manager and retrieve them easily in Ansible.

```yaml
"{{ lookup('google.cloud.gcp_secret', 'app-password', on_missing='skip')}}"
```

Start by studying the AWS secret lookup plugin linked in the issue and the google.cloud collection's existing lookup-plugin structure. Define the lookup entry point and Google Secret Manager interaction, then verify the documented example retrieves app-password and that on_missing='skip' behaves as requested.

Written by the indexing model from the issue text.