fix(sui-indexer): allow setting --gc-checkpoint-files=false

Previously I was unable to turn off checkpoint file garbage collection,
since the default value was `true`, the `--gc-checkpoint-files` flag
didn't expect any values and the default `clap` action was to set the
value to `true` when the flag was present. I've tried all of:

- `--gc-checkpoint-files 0`
- `--gc-checkpoint-files false`
- `--gc-checkpoint-files=0`
- `--gc-checkpoint-files=false`

This changes that so that the `false` value is accepted by the argument.
It is based on this [comment](clap-rs/clap#1649 (comment))

crates/sui-indexer/src/config.rs

@@ -142,7 +142,13 @@ pub struct IngestionConfig {
 
     /// Whether to delete processed checkpoint files from the local directory,
     /// when running Fullnode-colocated indexer.
-    #[arg(long, default_value_t = true)]
+    #[arg(
+        long,
+        default_value_t = true,
+        default_missing_value = "true",
+        num_args = 0..=1,
+        require_equals = false,
+    )]
     pub gc_checkpoint_files: bool,
 }