HTTPBackendRef RequestHeaderModifier Conformance Test

[dprotaso]

What type of PR is this? /kind cleanup /kind test /area conformance

What this PR does / why we need it:

This adds test coverage for an extended feature - Supporting request header modification on the HTTPBackendRef

Which issue(s) this PR fixes: N/A

Does this PR introduce a user-facing change?:

NONE

[dprotaso]

adding approvers /assign @arkodg @mlavacca @sunjayBhatia

adding reviewers /cc @LiorLieberman @michaelbeaumont @Xunzhuo

[dprotaso]

Conformance reviewers/approvers - can I get review please @arkodg @mlavacca @sunjayBhatia @michaelbeaumont @LiorLieberman @Xunzhuo

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: arkodg, dprotaso

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Needs approval from an approver in each of these files:

• ~~conformance/OWNERS~~ [arkodg]

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[dprotaso]

Looks good to me, which implementation did you test it with? /lgtm

This passes with Contour and Istio