MDEV-32013 Add Field::val_lex_string_strmake()

There are two functions to extract a Field::val_str() value
as a LEX_STRING or LEX_CSTRING pointing to the data allocated on a MEM_ROOT:

  char *get_field(MEM_ROOT *mem, Field *field);
  bool get_field(MEM_ROOT *mem, Field *field, class String *res);

The first function requires strlen() calls to make a LEX_CSTRING/LEX_STRING.
The second function requires a redundant String buffer,
which is used only as a temporary proxy value pointing to a MEM_ROOT fragment
(and does not use any String dynamic allocation methods).

This patch add a native way to extract a Field::val_str() value
as a LEX_STRING or LEX_CSTRING pointing to a MEM_ROOT fragment.
It helps to remove redundant strlen() calls and redundant String buffers.

- Adding a new method:

    LEX_STRING Field::val_lex_string_strmake(MEM_ROOT *mem);

- Reusing the new method Field::val_lex_string_strmake() in;

    bool get_field(MEM_ROOT *mem, Field *field, String *res);

  Also, moving it from table.cc to a static function in sql_help.cc.
  It is used in sql_help.cc only, and we don't want it to be reused
  in other parts of the code (to avoid redundant String buffers).

- Reusing the new method Field::val_lex_string_strmake() in this function:

    char *get_field(MEM_ROOT *mem, Field *field);

- Replacing get_field() to Field::val_lex_string_strmake() in these files:

    sql_plugin.cc  (redundant String buffers were removed)
    sql_udf.cc     (redundant strlen() calls were removed)

Note, this function:

   char *get_field(MEM_ROOT *mem, Field *field);

is still used in a number of files:

   event_data_objects.cc
   event_db_repository.cc
   sql_acl.cc
   sql_servers.cc

These remaining calls will be removed by separate patches,
and get_field() will be removed after that.

sql/table.h

@@ -3404,7 +3404,6 @@ bool check_period_name(const char *name);
 bool check_table_name(const char *name, size_t length, bool check_for_path_chars);
 int rename_file_ext(const char * from,const char * to,const char * ext);
 char *get_field(MEM_ROOT *mem, Field *field);
-bool get_field(MEM_ROOT *mem, Field *field, class String *res);
 
 bool validate_comment_length(THD *thd, LEX_CSTRING *comment, size_t max_len,
                              uint err_code, const char *name);