Add forwarded_attributes to per route config

[kyessenov]

Implements the API change istio/api#500

[googlebot]

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[kyessenov]

PTAL. This is a repeat of the old PR against master.

[googlebot]

A Googler has manually verified that the CLAs look good.

(Googler, please make sure the reason for overriding the CLA status is clearly documented in these comments.)

[qiwzhang]

/lgtm
/approve

[istio-testing]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: kyessenov, qiwzhang

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