add select hints for postgresql

[yy-at-here]

this PR is related to the isse: #641.

Summary

• Add support for pg_hint_plan optimizer hints in PostgreSQL SELECT queries
• Hints are rendered as /*+ ... */ comments before the SELECT clause
• Supported hint categories:
  • Scan hints: SeqScan, NoSeqScan, IndexScan, NoIndexScan, IndexOnlyScan, NoIndexOnlyScan, BitmapScan, NoBitmapScan, TidScan, NoTidScan
  • Join hints: NestLoop, NoNestLoop, HashJoin, NoHashJoin, MergeJoin, NoMergeJoin
  • Join order: Leading
  • Row estimation: Rows
  • Parallel: Parallel
  • GUC: Set

Example

psql.Select(
    sm.SeqScan("t1"),
    sm.NestLoop("t1", "t2"),
    sm.Columns("id", "name"),                                                                                                                                                                                                                                                            
    sm.From("t1"),
    sm.InnerJoin("t2").On(psql.Quote("t1", "id").EQ(psql.Quote("t2", "id"))),                                                                                                                                                                                                            
)                                                                                                                                                                                                                                                                                        
// => /*+ SeqScan(t1) NestLoop(t1 t2) */ SELECT id, name FROM t1 INNER JOIN t2 ON ("t1"."id" = "t2"."id")

How to test

• Unit tests added in TestSelectHints covering single hint, multiple hints, index scan with multiple indexes, leading hint, and set hint