Add lb route extension resource

[modular-magician]

Adds the new google_network_services_lb_route_extension resource.

Part of: #17491
Fixes: #17372

Release Note Template for Downstream PRs (will be copied)

`google_network_services_lb_route_extension`

networkservices: made required fields `extension_chains.extensions.authority ` and `extension_chains.extensions.timeout` optional in resource `google_network_services_lb_traffic_extension`

networkservices: made field `load_balancing_scheme` immutable in resource `google_network_services_lb_traffic_extension`, as in-place updating is always failing

networkservices: removed unsupported load balancing scheme `LOAD_BALANCING_SCHEME_UNSPECIFIED` from the field `load_balancing_scheme` in resource `google_network_services_lb_traffic_extension`

Derived from GoogleCloudPlatform/magic-modules#10119

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