Add experimental complete-blob-backfill flag (#7751)

A different (and complementary) approach for:

- https://github.com/sigp/lighthouse/issues/5391


  This PR adds a flag to set the DA boundary to the Deneb fork. The effect of this change is that Lighthouse will try to backfill _all_ blobs.

Most peers do not have this data, but I'm thinking that combined with `trusted-peers` this could be quite effective.


Co-Authored-By: Michael Sproul <michael@sigmaprime.io>

beacon_node/src/cli.rs

@@ -401,6 +401,16 @@ pub fn cli_app() -> Command {
                 .help_heading(FLAG_HEADER)
                 .display_order(0)
         )
+        .arg(
+            Arg::new("complete-blob-backfill")
+                .long("complete-blob-backfill")
+                .help("Download all blobs back to the Deneb fork epoch. This will likely result in \
+                       the node banning most of its peers.")
+                .action(ArgAction::SetTrue)
+                .help_heading(FLAG_HEADER)
+                .display_order(0)
+                .hide(true)
+        )
         .arg(
             Arg::new("enable-private-discovery")
                 .long("enable-private-discovery")