Allow subordinate CA data sources in all states

[modular-magician]

Resolves #12432

Currently, attempting to use a google_privateca_certificate_authority for a SUBORDINATE CA in any state other than AWAITING_USER_ACTIVATION is impossible, because the Google API can only fetch the CSR in that state (reference). This prevents some really common use cases, like pulling an existing subordinate CA to issue leaf certs.

With this change, pulling a subordinate CA in any state other than AWAITING_USER_ACTIVATION will result in pem_csr simply being unset. If the subordinate CA is in AWAITING_USER_ACTIVATION and the CSR fetch fails, an error will still be thrown as expected.

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.

Release Note Template for Downstream PRs (will be copied)

privateca: fixed an issue that blocked subordinate CA data sources when `state` was not `AWAITING_USER_ACTIVATION`

Derived from GoogleCloudPlatform/magic-modules#6496

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