Add CEL validation test for `targetRef` in ClientSettingsPolicy #14339

Job	Run time
Checks and variables	1m 51s
NJS Unit Tests	12s
Unit Tests	1m 13s
CEL Tests	2m 15s
Build Binary	0s
Build Plus images	0s
Build OSS images	0s
Functional tests	0s
Conformance tests	0s
Helm Tests	0s
Package and Publish Helm Chart	0s
	5m 31s

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