Add Rust FMT

[insipx]

Bunch of big PRs were merged and work seems about to go full steam ahead on MLS, so I figured it a good time to add rustfmt. Option description here. Other popular options are use_small_heuristics/fn_args_layout.

I configured some basic options, mostly w.r.t comments and organizing of module imports. The most controversial is probably module import method, but the 'StdExternalCrate' option should organize things in three groups:

use alloc::alloc::Layout;
use core::f32;
use std::sync::Arc;

use broker::database::PooledConnection;
use chrono::Utc;
use juniper::{FieldError, FieldResult};
use uuid::Uuid;

use super::schema::{Context, Payload};
use super::update::convert_publish_payload;
use crate::models::Event;

which makes it nice & easy to see whether imports are internal, external, or part of the standard library. I believe it is also aligns with auto-import in rust-analyzer.

[insipx]

lets hold off on merge until #300