Add filesystem for accessing GCP Secret Manager

[hairyhenderson]

This is effectively the same as the awssmfs package (#28), but for GCP Secret Manager. I propose gcp+sm as the URL scheme.

See hairyhenderson/gomplate#788

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[github-actions]

This issue is stale because it has been open for 60 days with no activity. Remove stale label or comment or this will be automatically closed in a few days.

[github-actions]

This issue is stale because it has been open for 60 days with no activity. Remove stale label or comment or this will be automatically closed in a few days.