support variable of `default_collation_for_utf8`

## Feature Request

My application originally used a MySQL database. When creating tables with the utf8 charset, the expected collation was utf8_general_ci.

Since the default collation for utf8 in TiDB is utf8_bin, I need to modify either the business logic or the CREATE TABLE statements.

However, because a large number of applications are involved, modifying the business logic or the CREATE TABLE statements is difficult to execute.

Therefore, I hope to add a system variable to set the default collation for the utf8 charset.

TiDB already supports default_collation_for_utf8mb4, so the same logic can be implemented.