JSON reader doesn't support scalar-to-list promotion, even though schema inference does

**Describe the bug**
Whilst doing some hacking on the JSON schema inference logic, I noticed that there is logic to "promote" scalars to lists when the two are mixed. For example, if one row has `"foo": [1, 2, 3]` and another has `"foo": 4`, then "foo" will be inferred to have type `List(Int64)` rather than reporting a mismatch.

However, this behaviour doesn't appear to be present in the actual JSON reader.

**To Reproduce**
I added this simple test case that uses the already available "mixed_arrays.json" test file:

```rust
#[test]
fn test_json_mixed_arrays() {
let reader = read_file("test/data/mixed_arrays.json", None);
reader.collect::, _>>().unwrap();
}
```

It returns the following error:
```text
thread 'reader::tests::test_json_mixed_arrays' (2600) panicked at arrow-json\src\reader\mod.rs:2047:47:
called `Result::unwrap()` on an `Err` value: JsonError("whilst decoding field 'b': expected [ got 4")
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
test reader::tests::test_json_mixed_arrays ... FAILED
```

**Expected behavior**
Either:
* The schema inference fails (i.e. no scalar promotion logic), or
* The test succeeds

Personally, I would prefer the former, as this "scalar-to-list promotion" logic feels like an unconventional and unexpected feature to have for a JSON parser.