StringIndexer supports multiple ways of label ordering

StringIndexer maps labels to numbers according to the descending order of label frequency. Other types of ordering (e.g., alphabetical) may be needed in feature ETL. For example, the ordering will affect the result in one-hot encoding and RFormula. Propose to support other ordering methods and we add a parameter stringOrderType that supports the following four options:

• 'freq_desc': descending order by label frequency (most frequent label assigned 0)
• 'freq_asc': ascending order by label frequency (least frequent label assigned 0)
• 'alphabet_desc': descending alphabetical order
• 'alphabet_asc': ascending alphabetical order