openprotocol changed value behavior compatible with new collation

[pepezzzz]

Feature Request

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

In the default configuration, the Row Changed Event of TiCDC Open Protocol output in a replication task only contains the changed value, not the value before the change. Therefore, the output value neither supports the new collation framework introduced in TiDB v4.0 .

Enduser already build some flink application handling downstream openprotocol format data with enable-old-value = false. If these applications move to a TiDB cluster downstream with new collation enabled，that will make a compatible issue. Flink application can not deal with the p column properly which comes from enable-old-value , but TiCDC in TiDB cluster with new collation enabled always create enable-old-value task.

Describe the feature you'd like:

the search old value procedure always included by ticdc task，enable-old-value configuration only control the old value output behavior not the search behavior. Then ,this configuration can compatible with new collation.

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy:

[amyangfei]

I'm not quite understand the requirement, do you mean we should add another old-value switch, which can turn on old-value when receiving data from TiKV, and turn off the old-value output field in the open protocol

Besides enable-old-value is controlled in changefeed config, even for a upstream with new CI collation enabled, the old-value feature can be turned off (expect for two scenarios, canal and maxwell will enable old value by default, even the enable-old-value is false in the changefeed config)

[hi-rustin]

Duplicate of #2301