Implement `sql` type interfaces for system types

`sql.StringType` was implemented for `types.systemStringType` in dolthub/go-mysql-server#3438

We need to do this for other system types (e.g., implementing `sql.NumberType` for `systemIntType`, etc)

This is important to make sure joining/indexing on system variables works correctly (related to #10316)

Start with the prior sql.StringType implementation for types.systemStringType described in dolthub/go-mysql-server#3438, then inspect systemIntType and the other system types. Confirm which sql interfaces each type needs, and verify that joins and indexing on system variables work correctly when the interfaces are implemented.

Written by the indexing model from the issue text.