feat(core): enhance the manifest serizlization and deserialization

[goldmedal]

Description

This PR enhances some issues about the MDLserizlization and deserialization.

Optional Field

Some fields are optional and can be absent in the MDL. For example, in the model definition, the user only needs to provide one of tableReference, refSql, or baseObject.

{
    "name": "orders",
    "tableReference": {
        "table": "orders"
    },
    "refSql": null  // optional
}

Empty expression

In the expression field of the Column, it could be an empty string. We should read it as a null value.

{
    "name": "o_orderkey",
    "type": "int",
    "expression": ""
}

Read Bool from integer

In the MDL provided by Wren AI, the boolean value could be 0 or 1. This PR also backward supports it.

{
    "name": "o_orderkey",
    "isCalculated": 0
}

Fix the name of NotNull

Fix no_null -> not_null.

[grieve54706]

LGTM. Thanks @goldmedal.