Select aggregate functions

The library supports the aggregation functions for the queries:

use firestore::*;
use serde::{Deserialize, Serialize};
#[derive(Debug, Clone, Deserialize, Serialize)]
struct MyAggTestStructure { counter: usize }
const TEST_COLLECTION_NAME: &str = "test";
async fn example(db: FirestoreDb) -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
let objs: Vec<MyAggTestStructure> = db
    .fluent()
    .select()
    .from(TEST_COLLECTION_NAME)
    .aggregate(|a| a.fields([a.field(path!(MyAggTestStructure::counter)).count()]))
    .obj()
    .query()
    .await?;
let _ = objs;
Ok(())
}