Add body to http.Request and roundTripper.request to extend conformance testutil ability to send request with body.

[zetxqx]

What type of PR is this?

/area conformance-test

What this PR does / why we need it:
The conformance testutil roundTripper and http.request does not allow to pass a request body into it.
This PR added the ability to set the request body which is needed in inference-gateway-extention conformance test PR#961

Which issue(s) this PR fixes:

Fixes # N/A

Does this PR introduce a user-facing change?:

NONE

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[k8s-ci-robot]

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[robscott]

Thanks @zetxqx!

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: robscott, zetxqx

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• conformance/OWNERS [robscott]

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[robscott]

/ok-to-test