[Improve] support flex column update for flink sql

[JNSimba]

Proposed changes

Support flex column update for flink sql

Checklist(Required)

1. Does it affect the original behavior: (Yes/No/I Don't know)
2. Has unit tests been added: (Yes/No/No Need)
3. Has document been added or modified: (Yes/No/No Need)
4. Does it need to update dependencies: (Yes/No)
5. Are there any changes that cannot be rolled back: (Yes/No)

Further comments

If this is a relatively large or complex change, kick off the discussion at dev@doris.apache.org by explaining why you chose the solution you did and what alternatives you considered, etc...

[JNSimba]

In FlinkSQL scenarios, when concatenating RowData, it's essential to adhere to the Flink table's schema. For example, if the Flink schema is [id, a, b, c, d], even if the upstream only sends id and a, the RowData will still be concatenated according to the schema, resulting in displayed concatenations of c=null and d=null. This would then modify the data in c and d.

However, if the Flink schema only defines id and a, then partial column updates can achieve the desired result.

But if the upstream might send [id, a], [id, b], [id, c], then you must use DataStream's SimpleStringSerializer to write the data in JSON format.

Furthermore, it's crucial to ensure that the columns passed from the upstream are truly only these few columns. For instance, it's uncertain whether FlinkSQL or Kafka will add null columns to align with the source schema.