Allow specifying global L7 ILB in dns_record_set routing policy

[modular-magician]

This resource currently only allows specifying regional L4 ILB targets to health check in routing policies (see the GoogleCloudPlatform/magic-modules#6665). We will add support for specifying regional L7 ILBs (regional internal HTTP(S) load balancers) as well.

b/293348842

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Release Note Template for Downstream PRs (will be copied)

dns: added `globalL7ilb` enum support to the `routing_policy.load_balancer_type` field in `google_dns_record_set`

Derived from GoogleCloudPlatform/magic-modules#8470

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