Add ability to set `stopFetchingTriggersCompletionHandler` on `GTMSessionFetcherService` #295

Job	Run time
pod-lib-lint (ios, Debug)	8m 8s
pod-lib-lint (ios, Debug)	8m 8s
pod-lib-lint (ios, Release)	8m 47s
pod-lib-lint (ios, Release)	8m 47s
pod-lib-lint (macos, Debug)	5m 30s
pod-lib-lint (macos, Debug)	5m 30s
pod-lib-lint (macos, Release)	5m 27s
pod-lib-lint (macos, Release)	5m 38s
pod-lib-lint (tvos, Debug)	5m 46s
pod-lib-lint (tvos, Debug)	5m 46s
pod-lib-lint (tvos, Release)	5m 55s
pod-lib-lint (tvos, Release)	5m 55s
pod-lib-lint (visionos, Debug)	9m 25s
pod-lib-lint (visionos, Debug)	9m 25s
pod-lib-lint (visionos, Release)	8m 48s
pod-lib-lint (visionos, Release)	8m 48s
pod-lib-lint (watchos --skip-tests, Debug)	49s
pod-lib-lint (watchos --skip-tests, Debug)	49s
pod-lib-lint (watchos --skip-tests, Release)	52s
pod-lib-lint (watchos --skip-tests, Release)	52s
	1h 59m 5s

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