[FEATURE] Support Enum as a basic type

[badmonster0]

Here are all the dataType supported in CocoIndex: https://cocoindex.io/docs/core/data_types

We want to add Enum Type. #217 is an example.

Representations in different places

• We can use a Arc<str> in BasicValue in Rust.
• We can use Enum in Python.
• Use string in different storages like Postgres
• Please also update conversion logic in [convert.rs]

Other potential code that needs to be updated

• Methods related BasicValueType, for serializing, deserializing, etc.
• BasicValue::from_json(). It needs to updated.
• JsonSchemaBuilder
• Rust<->Python value conversion logic py/convert.rs
• Python SDK code to update: typing.py, convert.py
  Please also the test test_convert.py
• Update the document data_types.mdx

(it may not be a fully list, feel free to update more files as needed.

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