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

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  Fixed [Feature Request] Unify loaders interfaces #1589
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