Add new data source google_compute_instance_template

[modular-magician]

Fixes #365.

Adds a new data source for instance templates. Allows to retrieve instance templates either by name or using a filter (allowing for name regex using e.g. filter = "name eq .*-${var.name}-tpl-.*"). most_recent orders (client side, as not supported by API) retrieved instances by creation timestamp and retrieves the latest one.

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Release Note Template for Downstream PRs (will be copied)

`google_compute_instance_template`

Derived from GoogleCloudPlatform/magic-modules#4362

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