add .timer command with fine grained statistics about limbo

```
Limbo v0.0.19-pre.4
Enter ".help" for usage hints.
limbo> .timer on
limbo> select count(1) from users;
┌───────────┐
│ count (1) │
├───────────┤
│     10000 │
└───────────┘
Command stats:
----------------------------
total: 35 ms (this includes parsing/coloring of cli app)

query execution stats:
----------------------------
Execution: avg=16 us, total=33 us
I/O: avg=123 ns, total=3 us
limbo> select 1;
┌───┐
│ 1 │
├───┤
│ 1 │
└───┘
Command stats:
----------------------------
total: 282 us (this includes parsing/coloring of cli app)

query execution stats:
----------------------------
Execution: avg=2 us, total=4 us
I/O: No samples available
```

cli/commands/args.rs

@@ -106,3 +106,15 @@ pub struct LoadExtensionArgs {
     #[arg(add = ArgValueCompleter::new(PathCompleter::file()))]
     pub path: String,
 }
+
+#[derive(Debug, ValueEnum, Clone)]
+pub enum TimerMode {
+    On,
+    Off,
+}
+
+#[derive(Debug, Clone, Args)]
+pub struct TimerArgs {
+    #[arg(value_enum)]
+    pub mode: TimerMode,
+}