feat: Migrate `plugin/source/slack` to `github.com/cloudquery/plugin-sdk/v2`

### Which problem is this feature request solving?

Part of https://github.com/cloudquery/cloudquery/issues/9846

### Describe the solution you'd like

Upgrade the code.

### Pull request (optional)

- [X] I can submit a pull request