Add ability to set custom column types in the gorm tag, so things such as lengths can be added.

[RParkerHp]

This allows customisation in the same way as already implemented for data types via the WithDataTypeMap method.

• Do only one thing
• Non breaking API changes
• Tested

What did this pull request do?

Adds the ability to customise the gorm tag's type field to add, for example, lengths.
This allows gorm:"type:char" to be set to gorm:"type:char(32)" (which isn't done by default for MS SQL Server, and possibly others).

User Case Description

An example of how this can be used is:

var columnMap = map[string]func(columnType gorm.ColumnType) (dataType string){
    "varchar":  getColumnTypeAndSize,
    "nvarchar": getColumnTypeAndSize,
    "nchar":    getColumnTypeAndSize,
    "char":     getColumnTypeAndSize,
}

cfg.WithColumnTypeMap(columnMap)

func getColumnTypeAndSize(columnType gorm.ColumnType) (dataType string) {
    typ, ok := columnType.ColumnType()
    if ok {
        len, ok := columnType.Length()
        if ok && len > 0 {
            return fmt.Sprintf("%v(%v)", typ, len)
        }
        return typ
    }
    return ""
}```