Adding hint text for a query SQL

[xJoeWoo]

Adds the ability to mutate the query SQL with hint text. Besides, adjusted AbstractQuery.copyTo function to public visibility and open to make extending the Query class easier.

[xJoeWoo]

This solution has its limitation: only supports Query statement. If the hint needs to be added to all statements, for example, used for tracking, another mechanism should be added to Statement class, or the StatementInterceptor class.

I recommend making StatementInterceptor more powerful, like intercept then editing the statement, or even the final SQL string. This makes updating the updateTime column(which should be updated every time the row changes) much much simpler, no need to assign updateTime column everywhere, and no worry about forgeting to update this column.

interface StatementInterceptor {

    // add this function, call in the first line of org.jetbrains.exposed.sql.statements.Statement#executeIn
    fun beforeArgument(transaction: Transaction, statement: Statement<*>) {}

    // even mutating the SQL, useful for adding tracking hints
    fun beforeStatementPrepared(transaction: Transaction, sql: String): String = sql

}

class UpdateTimeColumnType : ColumnType<LocalDateTime>()

object UpdateTimeGlobalStatementInterceptor : GlobalStatementInterceptor {
    override fun beforeArgument(transaction: Transaction, statement: Statement<*>) {
        val updateStatement = statement as? UpdateStatement ?: return
        val table = updateStatement.targetsSet as? Table ?: return
        val column = table.columns.firstOrNull { it.columnType is UpdateTimeColumnType } as? Column<LocalDateTime> ?: return

        updateStatement[column] = LocalDateTime.now()
    }
}