chore: update ITDynamicRoutingHeaders to use new standalone Interceptor classes #8579

Job	Run time
lint	26s
build(8) except for gapic-generator-java	4m 40s
build(21) except self-service clients	13m 50s
build(8) for gapic-generator-java	11m 58s
build (11)	14m 59s
compatibility	1m 37s
build (17)	14m 56s
gapic-generator-java-bom	1m 48s
showcase (11)	4m 3s
showcase-clirr	3m 6s
showcase (17)	4m 9s
showcase-native	8m 20s
showcase (21)	4m 0s
dependency_analyzer_unit_test	13s
unmanaged_dependency_check	4m 59s
	1h 33m 4s

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