[devkit/utilities] Implement Unix timestamp helpers

Helpers for working with Stellar ledger timestamps.

**Functions:**
```rust
pub fn now_unix_ms() -> u64
pub fn unix_ms_to_datetime(ms: u64) -> DateTime<Utc>
pub fn datetime_to_unix_ms(dt: DateTime<Utc>) -> u64
pub fn ledger_close_time_to_unix_ms(close_time: u64) -> u64
```

**Files to touch:**
- `packages/devkit/src/utilities/time.rs`
- `packages/devkit/Cargo.toml` — add chrono dep