[ROUTE-EMITTER PR REVIEW]: Add protocol to route-emitter messages

[moleske]

This is to support http2 work cloudfoundry/routing-release#200

cloudfoundry/route-emitter#16

[cf-gitbot]

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[belinda-liu]

The route-emitter PR is also related to this PR for routing-info: cloudfoundry/routing-info#2