Update/delete preconditions

The library supports the preconditions:

use firestore::*;
use serde::{Deserialize, Serialize};
#[derive(Debug, Clone, Deserialize, Serialize)]
struct MyTestStructure {
    some_num: u64,
}
const TEST_COLLECTION_NAME: &str = "test";
async fn example(db: FirestoreDb) -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
let object_updated: MyTestStructure = db
    .fluent()
    .update()
    .fields(paths!(MyTestStructure::{some_num}))
    .in_col(TEST_COLLECTION_NAME)
    .precondition(FirestoreWritePrecondition::Exists(true))
    .document_id("test-1")
    .object(&MyTestStructure { some_num: 1 })
    .execute()
    .await?;
let _ = object_updated;
Ok(())
}