bug: datagen connector should not be used with ROW FORMAT JSON

[fuyufjh]

Describe the bug

dev=> create source ss (id int) with ('connector' = 'datagen');
ERROR:  sql parser error: Expected ROW, found: ;

But this works:

dev=> create source ss (id int) with ('connector' = 'datagen') ROW FORMAT JSON;
CREATE_SOURCE

To Reproduce

See above.

Expected behavior

datagen connector should not be used with ROW FORMAT JSON

Additional context

[fuyufjh]

@shanicky PTAL 🥰

[shanicky]

The connector crate only generates raw bytes messages that need to be converted into chunks by the source parser, so this is a problem for this internal connector.
Let's figure it out.

cc @tabVersion

[tabVersion]

as I suggested in the very beginning, datagen should be at source level rather than connector. Moving to source level can prevent CPU resource waste (serializing generated data to json then deserializing in connector source).

[shanicky]

fixed in #7612