Add support for GroupByAppend

[ashishkf]

Is your feature request related to a problem? Please describe.
Unlike Select, Order, GroupBy have only GroupBy and SetGroupBy but not GroupByAppend.
Without this, we cannot add additional group by clauses - we need to get existing ones, do the append outside and then call SetGroupBy.

Describe the solution you'd like
A method on selectClauses GroupByAppend as follows:

func (c *selectClauses) GroupByAppend(cl ColumnListExpression) SelectClauses {
     ret := c.clone()
    ret.groupBy = ret.groupBy.Append(cl.Columns()...)
   return ret
}

Also, corresponding method on SelectDataset as follows:

func (sd *SelectDataset) GroupByAppend(groupBy ...interface{}) *SelectDataset {
   return sd.copy(sd.clauses.GroupByAppend(exp.NewColumnListExpression(groupBy...)))
}

Dialect

• postgres
• mysql
• sqlite3

[ashishkf]

#276

[doug-martin]

Released in v9.14.0 thank you for your PR!