Split the code for CHARACTER_OCTET_LENGTH and CHARACTER_MAXIMUM_LENGTH into methods in Field

The code in sql_show.cc displaying INFORMATION_SCHEMA.COLUMNS.CHARACTER_OCTET_LENGTH and INFORMATION_SCHEMA.COLUMNS.CHARACTER_MAXIMUM_LENGTH:

• tests field->type() and field->real_type() for various hard-coded constant MYSQL_TYPE_XXX
• uses max_display_length() with special tricks for the BLOB data types, e.g.:

  uint32 octet_max_length= field->max_display_length();

  if (is_blob && octet_max_length != (uint32) UINT_MAX32)

  octet_max_length /= field->charset()->mbmaxlen;

  where the test for UINT_MAX32 is a workaround to address a special behavior of Field_blob::max_display_length() for LONGBLOB.

• Uses has_charset() to detect data types which need NULL to be displayed in the mentioned I_S columns.

This is non friendly to pluggable data types.

Also, this makes it hard to fix bugs in max_display_length().

Under terms of this task we'll move the code in sql_show.cc into a new virtual method in Field:

  virtual Information_schema_character_attributes

    information_schema_character_attributes();

where Information_schema_character_attributes is a new class with approximately this structure:

class Information_schema_character_attributes

{

  uint32 m_octet_length;

  uint32 m_char_length;

  bool m_has_octet_length;

  bool m_has_char_length;

public:

  .. constructors and access methods ..

}