indexer-alt: --skip-watermark

## Description

Add a flag to skip writing to the watermark table. This would be useful
when an indexer is running in parallel with another indexer and it's
just not even productive to try writing to the watermarks table, because
it will always be behind.

## Test plan

Tested locally.

crates/sui-indexer-alt/src/handlers/mod.rs

@@ -96,6 +96,10 @@ pub struct CommitterConfig {
         value_parser = |s: &str| s.parse().map(Duration::from_millis),
     )]
     commit_interval: Duration,
+
+    /// Avoid writing to the watermark table
+    #[arg(long)]
+    skip_watermark: bool,
 }
 
 /// A batch of processed values associated with a single checkpoint. This is an internal type used
@@ -540,7 +544,7 @@ fn committer<H: Handler + 'static>(
                         "Gathered watermarks",
                     );
 
-                    if watermark_needs_update {
+                    if !config.skip_watermark && watermark_needs_update {
                         let guard = metrics
                             .watermark_commit_latency
                             .with_label_values(&[H::NAME])