Implement Envoy-based load balancing schemes

[jawnsy]

Add configuration for load_balancing_scheme setting in forwarding rules and backend service configuration.

Closes: terraform-google-modules/terraform-google-lb-http#257

Signed-off-by: Jonathan Yu [email protected]

[comment-bot-dev]

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[jawnsy]

@bharathkkb Hey, sorry for the ping, just wanted to make sure that you saw this. Is anything else needed to get this completed and merged?

[bharathkkb]

Tests seemed to be failing due to version bump in https://github.com/terraform-google-modules/terraform-google-lb-http/pull/281, removed explicit constraint in the example.