* Updates ImpersonationConfig in rest/config.go to include UID
attribute, and pass it through when copying the config
* Updates ImpersonationConfig in transport/config.go to include UID
attribute
* In transport/round_tripper.go, Set the "Impersonate-Uid" header in
requests based on the UID value in the config
* Update auth_test.go integration test to specify a UID through the new
rest.ImpersonationConfig field rather than manually setting the
Impersonate-Uid header
Signed-off-by: Margo Crawford <margaretc@vmware.com>
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