json: support parsing and serializing of strings with char arrays

Support parsing and serializing of struct fields that are defined as a
char array.

Use the token JSON_TOK_STRING_BUF to parse and serialize a string for a
char array, for example:

struct foo {
  const char *str;
  char str_buf[30];
};

struct json_obj_descr foo_descr[] = {
  JSON_OBJ_DESCR_PRIM(struct foo, str, JSON_TOK_STRING),
  JSON_OBJ_DESCR_PRIM(struct foo, str_buf, JSON_TOK_STRING_BUF),
};

The struct 'json_obj_descr' now has an additional union member 'field'
to store the size of the struct field, which is used with the token
'JSON_TOK_STRING_BUF' to determine the element size.

Fixes: #65200
Signed-off-by: Christoph Winklhofer <cj.winklhofer@gmail.com>