Explicit null value serialization

By default, all Option<> serialized as absent fields, which is convenient for many cases. However sometimes you need to have explicit nulls.

To help with that there are additional attributes implemented for serde(with):

• For any type:

use serde::{Deserialize, Serialize};
#[derive(Debug, Clone, Deserialize, Serialize)]
struct MyTestStructure {
#[serde(default)]
#[serde(with = "firestore::serialize_as_null")]
test_null: Option<String>,
}

• For Firestore timestamps attribute:

use firestore::*;
use serde::{Deserialize, Serialize};
#[derive(Debug, Clone, Deserialize, Serialize)]
struct MyTestStructure {
#[serde(default)]
#[serde(with = "firestore::serialize_as_null_timestamp")]
test_null: Option<FirestoreInstant>,
}