[Feature request] Can't make a PK AI "signed" it's forcing to create as "unsigned"

[thiago-shopper]

Environment

Knex version: 0.20.14
Database + version: mysql 5.8
OS: linux

Feature discussion / request

1. Explain what is your use case
   I want to create a table with PK AI but not "unsigned" and i checked out that is not possible.
   I think it is related to this part of the code.
   https://github.com/tgriesser/knex/blob/master/lib/dialects/mysql/schema/column.js#L30
   ColumnCompiler_MySQL.prototype.increments = 'int unsigned not null auto_increment primary key';

2. Explain what kind of feature would support this
   I think it should has a way to force to the column to be signed.

3. Give some API proposal, how the feature should work
   table.increments('id').signed().primary()