json: support parsing and serializing of char arrays and integers (int8_t, uint8_t, etc.)

[cjwinklhofer]

Support parsing and serializing of char arrays and integers

By extending the struct 'json_obj_descr' with an additional union member 'field' that stores the size of the struct field it is possible to serialize and parse char arrays, integers with different width and also enum types. Added three new tokens:

• JSON_TOK_STRING_BUF: for char arrays
• JSON_TOK_INT: "generic" signed integer types (int8_t, int16_t, int32_t, int64_t)
• JSON_TOK_UINT: "generic" unsigned integer types (uint8_t, uint16_t, uint32_t, uint64_t)

Strings with char arrays

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

Integers of different width

Use the token JSON_TOK_INT or JSON_TOK_UINT to parse and serialize integers of different types - for example:

struct foo {
  int64_t i64;
  uint32_t u32;
  int16_t i16;
  uint8_t u8;
};

struct json_obj_descr foo[] = {
  JSON_OBJ_DESCR_PRIM(struct foo, i64, JSON_TOK_INT),
  JSON_OBJ_DESCR_PRIM(struct foo, u32, JSON_TOK_UINT),
  JSON_OBJ_DESCR_PRIM(struct foo, i16, JSON_TOK_INT),
  JSON_OBJ_DESCR_PRIM(struct foo, u8, JSON_TOK_UINT),
};

Parsing and serializing enums

enum unsigned_enum { UA=0, UB=1, UC=2 };
enum signed_enum { SA=-1, SB=0, SC=1 };

struct foo {
  enum unsigned_enum u;
  enum signed_enum s;
};

struct json_obj_descr foo[] = {
  JSON_OBJ_DESCR_PRIM(struct foo, u, JSON_TOK_UINT),
  JSON_OBJ_DESCR_PRIM(struct foo, s, JSON_TOK_INT),
};

Fixes: #65200