fix: add migration to modify chatflowconfig to longtext for mysql

[sickOscar]

Using big CSV files as document loaders my break mysql chat flow because of the TEXT size of the column and base64 conversion.

Here a migration has been added to change the column type from TEXT to LONGTEXT (4GB of max data)

[HenryHengZJ]

@sickOscar thanks for the fix! However with this PR merged and released, everything is now stored under a local file path instead of a base64 string, hence this fix is no longer needed

[HenryHengZJ]

having said that, if you still have a lot of nodes and using MySQL, you might still face the issue. See https://github.com/FlowiseAI/Flowise/issues/2159.

So we can still proceed with this change

[HenryHengZJ]

closing as https://github.com/FlowiseAI/Flowise/pull/2908 merged