feat: Unify all `loaders` interfaces

[JoyceXu02]

Description

Unified all loaders' interfaces and updated corresponding tests. I use BaseLoader as the basic abstraction with some standard components: init(config): Common configuration initialization load(source, **kwargs): Main abstract method for loading data supported_formats: Property to declare supported formats suggested by @Wendong-Fan

Checklist

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• [x] I have updated the tests accordingly (required for a bug fix or a new feature)
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• [ ] I have added examples if this is a new feature Fixed #1589 #1687 If you are unsure about any of these, don't hesitate to ask. We are here to help!