Fix custom metrics adapter OpenAPI v3 definitions

[bruno-espino]

Set OpenAPIV3Config with the same generated definitions used for OpenAPI v2.

Observed from adapter logs with gcr.io/gke-release/custom-metrics-stackdriver-adapter:v0.16.7-gke.0 after the API aggregation layer queried OpenAPI v3:

Failed to build OpenAPI v3 for group apis/custom.metrics.k8s.io/v1beta2, "cannot find model definition for io.k8s.metrics.pkg.apis.custom_metrics.v1beta2.MetricValueList..."
Failed to build OpenAPI v3 for group apis/custom.metrics.k8s.io/v1beta1, "cannot find model definition for io.k8s.metrics.pkg.apis.custom_metrics.v1beta1.MetricValueList..."
Failed to build OpenAPI v3 for group apis/external.metrics.k8s.io/v1beta1, "cannot find model definition for io.k8s.metrics.pkg.apis.external_metrics.v1beta1.ExternalMetricValueList..."

Local repro: leave OpenAPIV3Config nil, call cmd.Config() before WithCustomMetrics / WithExternalMetrics, then build v3 definitions for one of the model names above. custom-metrics-apiserver initializes a default v3 config at that point, before metric providers are registered, so the custom/external metric definitions are missing.

#1174 added the name mapping for OpenAPIConfig, but OpenAPIV3Config is still nil. This wires v3 to the same generated definitions and adds a regression test for the exact missing model names.

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