chore: expose the user set executor on TransportProvider #3581

Job	Run time
filter	1m 22s
downstream-compatibility (spring-cloud-gcp)	12m 25s
downstream-compatibility (java-bigtable)	9m 55s
downstream-compatibility (java-firestore)	3m 50s
downstream-compatibility (java-pubsub)	3m 59s
conditional-required-check	3s
	31m 34s

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