sql: allowing paving of multi-region zone config fields on initial SET PRIMARY REGION

[otan]

After #63129 merges, users with multi-region fields on zone configs set must individually discard all their zone configurations.

We should instead allow users to use the same session variable override_multi_region_zone_config to override and automatically pave these zone configurations

Follow up to #63071.

Jira issue: CRDB-6448

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