Makes `IonDataSource::is_streaming()` an associated const #3561

Job	Run time
Setup Build Matrix (true, ion-rust, windows, macos, ubuntu, al2-x86, al2-arm, windows, macos, ubu...	1s
Build and Test (windows, default)	6m 36s
Build and Test (windows, all)	7m 4s
Build and Test (windows, experimental-ion-hash)	5m 18s
Build and Test (windows, experimental)	6m 8s
Build and Test (macos, default)	1m 54s
Build and Test (macos, all)	2m 8s
Build and Test (macos, experimental-ion-hash)	2m 7s
Build and Test (macos, experimental)	2m 33s
Build and Test (ubuntu, default)	2m 19s
Build and Test (ubuntu, all)	4m 7s
Build and Test (ubuntu, experimental-ion-hash)	2m 12s
Build and Test (ubuntu, experimental)	2m 36s
Build and Test (al2-x86, default)	1m 35s
Build and Test (al2-x86, all)	1m 55s
Build and Test (al2-x86, experimental-ion-hash)	1m 48s
Build and Test (al2-x86, experimental)	1m 59s
Build and Test (al2-arm, default)	1m 39s
Build and Test (al2-arm, all)	1m 57s
Build and Test (al2-arm, experimental-ion-hash)	1m 43s
Build and Test (al2-arm, experimental)	2m 0s
Build and Test Confirmation	1s
	59m 40s

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