This patch adds a proper support for auxiliary fields containing `Option<T>`.
As of now, KV pairs containing `Some(x)` will be printed as just `x`,
while the ones with `None` won't be printed at all.

Example:

```rust
crate::audit!(
    message: "demo audit event",
    title: "example",
    severity: Low,
    test: Some(0u32),
);
```

```json
{
  "id": "1.0.1",
  "message": "demo audit event",
  "severity": "low",
  "time": "2023-12-06T17:16:51.085+0300",
  "title": "example",
  "test": "0"
}
```

```rust
crate::audit!(
    message: "demo audit event",
    title: "example",
    severity: Low,
    test: None::<i32>,
);
```

```json
{
  "id": "1.0.1",
  "message": "demo audit event",
  "severity": "low",
  "time": "2023-12-06T17:16:51.085+0300",
  "title": "example"
}
```