feat: add enable_relay to datapathv2 observability config

[aniekgul]

Adds in the enable_relay option into advanced_datapath_observability_config https://registry.terraform.io/providers/hashicorp/google/latest/docs/resources/container_cluster#advanced_datapath_observability_config

Defaulted to false.

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[Markieta]

Made a similar PR here with a few additional changes as needed.

[aniekgul]

Ah perfect, will close this off then