feat: define watermark on source

[fuyufjh]

Is your feature request related to a problem? Please describe.

See #6750 (comment) for context.

CREATE SOURCE Orders (
    user BIGINT,
    product STRING,
    order_time TIMESTAMP(3),
    WATERMARK FOR order_time AS order_time - INTERVAL '5' SECOND
) WITH ( ... );

Referred to the Flink document.

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Additional context

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