* Support per-replica tier override in modules/cloudsql-instance
Replicas previously ignored any caller-provided tier and silently
inherited the primary's tier. Add an optional `tier` field to the
`replicas` map and use `coalesce(each.value.tier, var.tier)` in the
replica resource so per-replica overrides take effect while
preserving the inherit-from-primary default.
* Update README
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Co-authored-by: Julio Castillo <jccb@google.com>