chore: expose the user set executor on TransportProvider #3619

Job	Run time
filter	1m 10s
downstream-compatibility (spring-cloud-gcp)	12m 30s
downstream-compatibility (java-pubsub)	4m 23s
downstream-compatibility (java-bigtable)	8m 29s
downstream-compatibility (java-firestore)	4m 52s
conditional-required-check	2s
	31m 26s

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