Add certificate template field to privateca Certificate

[modular-magician]

Now that privateca certificate template is present in TF.

Also adds test - which will only successfully run in CI since it depends on a static CA pool.

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

(note that tests will have to run on this PR overnight, since the privateca resources are hard to test and depend on a certificate pool that's set up statically).

Fixes #9792.

Release Note Template for Downstream PRs (will be copied)

`privateca`: Added `certificate_template` to `google_privateca_certificate`.

Derived from GoogleCloudPlatform/magic-modules#5135

[github-actions]

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