[EXECUTOR PR REVIEW]: Advertise HTTP/2 protocol in Envoy via ALPN

[mariash]

As part of the work to enable HTTP/2 support in Cloud Foundry we are making Envoy to advertise that it supports HTTP/2 and HTTP/1.1 via ALPN. Gorouter will use either one of them based on the route mapping. For more information see this issue

Related PR - cloudfoundry/executor#54

[cf-gitbot]

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