[lightning] automatically choose logical or physical import mode based on the table size.

## Feature Request

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

The logical import mode is significantly faster than the physical mode when importing small tables and the performance reverses on large tables. When a user wants to import databases with a large number of small tables together with a few huge tables, currently users have to split those tables into 2 batches by file size and then import them in 2 executions using different import modes. I would suggest supporting the hybrid import mode so that the tidb-lightning chooses the appropriate import mode by checking the file size.

**Describe the feature you'd like:**

**Describe alternatives you've considered:**

**Teachability, Documentation, Adoption, Migration Strategy:**