[flink] Support hint-based watermark configuration for Flink connector

[swuferhong]

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Motivation

Currently, Fluss Flink connector supports table-level watermark via WATERMARK FOR in DDL,
which is pushed down through Flink's SupportsWatermarkPushDown interface. However, there are
two common scenarios that are not well supported:

1. Legacy tables that already have a table-level WATERMARK definition but users want to
   disable it at query time without altering the table schema.
2. Ad-hoc watermark override where users want to specify a different watermark column or
   delay at query time via SQL hints, without modifying the table definition.

Solution

No response

Anything else?

No response

Willingness to contribute

• I'm willing to submit a PR!