support the variable of `tidb_default_auto_id_cache`

## Feature Request

My application originally used a MySQL database, and a large amount of business logic in the application relied on continuous and auto-incrementing primary keys.
Since the default value of TiDB AUTO_ID_CACHE is not 1, I need to modify the business logic or create table statements.
However, since a large number of applications are involved, modifying the business logic or creating table statements is difficult to execute. Therefore, I hope to add a system variable to set the default value of AUTO_ID_CACHE