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/sql_help.cc

@@ -69,6 +69,29 @@ enum enum_used_fields
 };
 
 
+/*
+  Allocate string field in MEM_ROOT and return it as String
+
+  SYNOPSIS
+    get_field()
+    mem         MEM_ROOT for allocating
+    field       Field for retrieving of string
+    res         result String
+*/
+
+static void get_field(MEM_ROOT *mem, Field *field, String *res)
+{
+  THD *thd= field->get_thd();
+  Sql_mode_instant_remove sms(thd, MODE_PAD_CHAR_TO_FULL_LENGTH);
+  LEX_STRING ls= field->val_lex_string_strmake(mem);
+  DBUG_ASSERT((!ls.str && !ls.length) || ls.str[ls.length] == '\0');
+  if (!ls.str)
+    res->length(0); // EOM
+  else
+    res->set((const char *) ls.str, ls.length, field->charset());
+}
+
+
 /*
   Fill st_find_field structure with pointers to fields