Support conversion between struct_pack and json without field names

### What happened + What you expected to happen
There are the following structures:
```C++
struct person {
int64_t id;
std::string name;
int age;
double salary;
};
```
Can be converted to:
```JSON
[1, "name", 1, 1.333333]

```
### Reproduction way
n/a

### Anything else
n/a

### Are you willing to submit a PR?
n/a

The issue names struct_pack and JSON conversion but provides no files, tests, or entry points. Begin by locating those components in the repository and comparing their existing conversion behavior; done means the shown person structure converts to the requested positional JSON array without field names.

Written by the indexing model from the issue text.