import into: auto detect data source format

[River2000i]

First-time contributors' checklist

• [x] I've signed Contributor License Agreement that's required for repo owners to accept my contribution.

What is changed, added or deleted? (Required)

https://github.com/pingcap/tidb/pull/59543

Which TiDB version(s) do your changes apply to? (Required)

Tips for choosing the affected version(s):

By default, CHOOSE MASTER ONLY so your changes will be applied to the next TiDB major or minor releases. If your PR involves a product feature behavior change or a compatibility change, CHOOSE THE AFFECTED RELEASE BRANCH(ES) AND MASTER.

For details, see tips for choosing the affected versions (in Chinese).

• [x] master (the latest development version)
• [ ] v9.0 (TiDB 9.0 versions)
• [ ] v8.5 (TiDB 8.5 versions)
• [ ] v8.4 (TiDB 8.4 versions)
• [ ] v8.3 (TiDB 8.3 versions)
• [ ] v8.1 (TiDB 8.1 versions)
• [ ] v7.5 (TiDB 7.5 versions)
• [ ] v7.1 (TiDB 7.1 versions)
• [ ] v6.5 (TiDB 6.5 versions)
• [ ] v6.1 (TiDB 6.1 versions)
• [ ] v5.4 (TiDB 5.4 versions)

What is the related PR or file link(s)?

• This PR is translated from:
• Other reference link(s): https://github.com/pingcap/docs/pull/21197

Do your changes match any of the following descriptions?

• [ ] Delete files
• [ ] Change aliases
• [ ] Need modification after applied to another branch
• [ ] Might cause conflicts after applied to another branch