Fix builder API headers (#7009)

Resolves https://github.com/sigp/lighthouse/issues/7000


  Set the accept header on builder to the correct value when requesting ssz.

This PR also adds a flag to disable ssz over the builder api altogether. In the case that builders/relays have an ssz bug, we can react quickly by asking clients to restart their nodes with the `--disable-ssz-builder` flag to force json. I'm not fully convinced if this is useful so open to removing it or opening another PR for it.

Testing this currently.

beacon_node/src/cli.rs

@@ -1460,6 +1460,15 @@ pub fn cli_app() -> Command {
                 .action(ArgAction::Set)
                 .display_order(0)
         )
+        .arg(
+            Arg::new("builder-disable-ssz")
+                .long("builder-disable-ssz")
+                .value_name("BOOLEAN")
+                .help("Disables sending requests using SSZ over the builder API.")
+                .requires("builder")
+                .action(ArgAction::SetTrue)
+                .display_order(0)
+        )
         .arg(
             Arg::new("reset-payload-statuses")
                 .long("reset-payload-statuses")