Istio gateways: Allow to specify namespace for TLS secret

[MartinWeindel]

What this PR does / why we need it:
Istio expects the TLS secret in the namespace of the ingress-gateway POD.
If a Gateway resource should be stored in a different namespace, the managed TLS secret will be created in the same namespace. With the added annotation cert.gardener.cloud/secret-namespace, it is possible to specify a different namespace. This annotation is only available for Istio Gateway resources and Services of type LoadBalancer.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Release note:

Istio gateways: Allow to specify namespace for TLS secret by annotation `cert.gardener.cloud/secret-namespace`.

[gardener-prow]

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