feat: support optional health_checks

[muskoxleader]

Currently, the serverless_negs module creates the google_compute_backend_service resource. This is where google_compute_health_check resources should be attached, however, this interface isn't exposed via var.backends.

This PR allows invokers of the serverless_negs module to use health checks.

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[imrannayer]

@muskoxleader Serverles NEGs dont support health checks.

https://cloud.google.com/load-balancing/docs/negs/serverless-neg-concepts#backend-service-limitations