Add feature flag to start from any beacon block in db (#15000)

* Add feature flag to start from any beacon block in db

The new feature flag called --sync-from takes a string that can take
values:

- `head` or
- a 0x-prefixed hex encoded beacon block root.

The beacon block root or the head block root has to be known in db and
has to be a descendant of the current justified checkpoint.

* Fix Bugs In Sync From Head (#15006)

* Fix Bugs

* Remove log

* missing save

* add tests

* Kasey review #1

* Kasey's review #2

* Kasey's review #3

---------

Co-authored-by: Nishant Das <nishdas93@gmail.com>

config/features/config.go

@@ -86,6 +86,9 @@ type Flags struct {
 
 	// AggregateIntervals specifies the time durations at which we aggregate attestations preparing for forkchoice.
 	AggregateIntervals [3]time.Duration
+
+	// Feature related flags (alignment forced in the end)
+	ForceHead string // ForceHead forces the head block to be a specific block root, the last head block, or the last finalized block.
 }
 
 var featureConfig *Flags
@@ -268,6 +271,10 @@ func ConfigureBeaconChain(ctx *cli.Context) error {
 		logEnabled(enableExperimentalAttestationPool)
 		cfg.EnableExperimentalAttestationPool = true
 	}
+	if ctx.IsSet(forceHeadFlag.Name) {
+		logEnabled(forceHeadFlag)
+		cfg.ForceHead = ctx.String(forceHeadFlag.Name)
+	}
 
 	cfg.AggregateIntervals = [3]time.Duration{aggregateFirstInterval.Value, aggregateSecondInterval.Value, aggregateThirdInterval.Value}
 	Init(cfg)