Bigtable: Update local gc_rules on read

[modular-magician]

Currently when GCPolicy remote resource is updated not on TF (Pantheon, gcloud CLI, etc.), the gc_rules string are not updated on read. This PR fixes the issue by parsing the GC policy into JSON string that TF understands and set it in the local gc_rules. This hopefully will resolve the drift issue we have seen in KCC.

The original PR: GoogleCloudPlatform/magic-modules#6415 was created by hoangpham95.

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Release Note Template for Downstream PRs (will be copied)

bigtable: added drift detection on `gc_rules` for `google_bigtable_gc_policy`

Derived from GoogleCloudPlatform/magic-modules#6526

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