Key idea:
1. Image Loading pipeline is based on `operation key`, when start/cancel it always need operation key
2. Stateful view has multiple operation key for different image property, we can use KeyPath as the hint to generate operation key
3. To make exists API compatible, we do not change much, however, in 6.0 we may do refactory and store all the loadState/operation/result into the single object, instead of 3
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