Support setting secrets with Secrets Manager

[christophersjchow]

Google introduced Secrets Manager support to functions in August last year. https://cloud.google.com/blog/products/serverless/cloud-functions-integrates-with-google-secret-manager

It would be great to be able to specify secrets within config to be pulled and set as env vars in functions.

[pgrzesik]

Hello @christophersjchow - thanks for proposing, we'd be happy to accept a PR if someone is interested. 🙌

[christophersjchow]

@pgrzesik i've raised 2 PRs, one here #287 and one for docs here serverless/serverless#10660

[christophersjchow]

Closed by #287