Step#2 MDEV-27896 Wrong result upon `COLLATE latin1_bin CHARACTER SET latin1` on the table or the database level

- Renaming Lex_charset_collation_st to
  Lex_exact_charset_extended_collation_attrs_st

- Renaming Lex_explicit_charset_opt_collate to
  Lex_exact_charset_opt_extended_collate

- Renaming their methods charset_collation() to charset_info(),
  so the name clearly tells that it returns CHARSET_INFO.
  Soon we'll have new classes (e.g. Lex_exact_collation) and
  methods returning Lex_exact_collation. So the old name would be
  confusing about the return type.