WIP: Support FeatureView as DataSource

[HaoXuAI]

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Since the DAG was introduced in the compute engine, the data source can now be a feature view.

Describe the solution you'd like
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Example:

bfv1 = BatchFeatureView(
source=FileSource(...)
)
bfv2 = BatchFeatureView(
source=bfv1,
...
)
bfv3 = BatchFeatureView(
source=[bfv1, bfv2],
...
)

Can use Single DataSource, Single FeatureView, or List of FeatureView as source.

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Additional context
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[HaoXuAI]

WIP