[feat][struct_pack][struct_json] support struct bit-field

example：
```
struct person {
std::string name;
int age;
int weight:16;/*bit-field*/
int height:16;
};
```

Start by reading the struct_pack and struct_json entry points and tracing how the example's fields are handled during serialization and deserialization. Determine the expected behavior for the bit-field members, then verify that both libraries support the example consistently and add or run the relevant coverage once the project identifies it.

Written by the indexing model from the issue text.