Add `IMPORT FOREIGN SCHEMA` options

[mkgrgis]

This is a draft for https://github.com/laurenz/oracle_fdw/issues/656. @laurenz , feel free to use and modify it. In this PR

• Add import_tables, import_views, import_materialized_views options in README file,
• Add a function for parsing boolean value of some option
• Add values parsing for import_tables, import_views, import_materialized_views options,
• Add import_tables, import_views, import_materialized_views in oracleGetImportColumn signature,
• Modify the main query in oracleGetImportColumn to more ISO:SQL style and add object_type. Final query version is

  SELECT col.table_name, col.column_name, col.data_type, col.data_type_owner,
     col.char_length, col.data_precision, col.data_scale, col.nullable,
     CASE WHEN primkey_col.position IS NOT NULL THEN 1 ELSE 0 END AS primary_key,
     obj.object_type      
  FROM all_tab_columns col
  LEFT JOIN     
   (SELECT con.table_name, cons_col.column_name, cons_col.position
      FROM all_constraints con, all_cons_columns cons_col
     WHERE con.owner = cons_col.owner
       AND con.table_name = cons_col.table_name
       AND con.constraint_name = cons_col.constraint_name
       AND con.constraint_type = 'P'
       AND con.owner = :nsp) primkey_col
  ON col.table_name = primkey_col.table_name
  AND col.column_name = primkey_col.column_name
  INNER JOIN ALL_OBJECTS obj
  ON col.table_name = obj.object_name
  AND col.owner = obj.owner
  WHERE col.owner = :nsp
  ORDER BY col.table_name, col.column_id;

  You can add AND object_type IN ('...', '...') in WHERE based on new flags and your preferred code style. Sorry for my not complete PR, I am just C beginner from sqlite_fdw.

[laurenz]

Thanks! I finished the implementation, polished the code and added regression tests.

[mkgrgis]

Thanks, @laurenz ! This was very interesting for me to see how my draft was modified and which code was merged.

[laurenz]

I a particularly grateful for the boolVal(). I could use it so make several parts of the code more readable. I had second thoughts and changed the import_* to skip_*, since I had the impression that would be more intuitive to use.

[mkgrgis]

Thanks to you, @laurenz for skip option conception. I'll discuss with author of firebird_fdw about implementation of similar options, now there is no special options about materialized views an tables. So, oracle_fdw will be a prototype for other implementations as usual ;-)