feat(networkservices): add DNS peering config to AgentGateway:

[modular-magician]

Adds the dnsPeeringConfig nested object under privateConnectivity.egress
on google_network_services_agent_gateway, allowing users to specify
domains, targetProject, and targetNetwork so the gateway resolves
queries via Cloud DNS in a peered VPC.

Beta Provider Test Log

==> Checking that code complies with gofmt requirements...
go vet
TF_ACC_REFRESH_AFTER_APPLY=1 TF_ACC=1 TF_SCHEMA_PANIC_ON_ERROR=1 go test ./google-beta/services/networkservices -v -run=TestAccNetworkServicesAgentGateway_networkServicesAgentGatewayFullExample -timeout 240m -ldflags="-X=github.com/hashicorp/terraform-provider-google-beta/version.ProviderVersion=acc"
=== RUN   TestAccNetworkServicesAgentGateway_networkServicesAgentGatewayFullExample
=== PAUSE TestAccNetworkServicesAgentGateway_networkServicesAgentGatewayFullExample
=== CONT  TestAccNetworkServicesAgentGateway_networkServicesAgentGatewayFullExample
--- PASS: TestAccNetworkServicesAgentGateway_networkServicesAgentGatewayFullExample (590.72s)
PASS
ok  	github.com/hashicorp/terraform-provider-google-beta/google-beta/services/networkservices	590.908s

GA Provider Test Log

==> Checking that code complies with gofmt requirements...
go vet
TF_ACC_REFRESH_AFTER_APPLY=1 TF_ACC=1 TF_SCHEMA_PANIC_ON_ERROR=1 go test ./google/services/networkservices -v -run=TestAccNetworkServicesAgentGateway_networkServicesAgentGatewayFullExample -timeout 240m -ldflags="-X=github.com/hashicorp/terraform-provider-google/version.ProviderVersion=acc"
=== RUN   TestAccNetworkServicesAgentGateway_networkServicesAgentGatewayFullExample
=== PAUSE TestAccNetworkServicesAgentGateway_networkServicesAgentGatewayFullExample
=== CONT  TestAccNetworkServicesAgentGateway_networkServicesAgentGatewayFullExample
--- PASS: TestAccNetworkServicesAgentGateway_networkServicesAgentGatewayFullExample (712.94s)
PASS
ok  	github.com/hashicorp/terraform-provider-google/google/services/networkservices	713.119s

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networkservices: added `dns_peering_config` field to `google_network_services_agent_gateway` resource

Derived from GoogleCloudPlatform/magic-modules#17545