Add EXTERNAL_MANAGED as load_balancing_scheme for google_compute_backend_service

[modular-magician]

This adds EXTERNAL_MANAGED as a load_balancing_scheme option for the backend service resource. It's one of the parts needed to add support for the new Global External HTTP(S) Load Balancer (the other part will require DCL changes to the global forwarding rule).

part of #10858

Tested with make testacc TEST=./google-beta TESTARGS='-run=TestAccComputeBackendService_backendServiceExternalManagedExample'

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compute: Added EXTERNAL_MANAGED as option for load_balancing_scheme in google_compute_backend_service resource

Derived from GoogleCloudPlatform/magic-modules#5596

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