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>

encoding/bytesutil/hex.go

@@ -22,12 +22,15 @@ func IsHex(b []byte) bool {
 // DecodeHexWithLength takes a string and a length in bytes,
 // and validates whether the string is a hex and has the correct length.
 func DecodeHexWithLength(s string, length int) ([]byte, error) {
+	if len(s) > 2*length+2 {
+		return nil, fmt.Errorf("%s is greather than length %d bytes", s, length)
+	}
 	bytes, err := hexutil.Decode(s)
 	if err != nil {
 		return nil, errors.Wrap(err, fmt.Sprintf("%s is not a valid hex", s))
 	}
 	if len(bytes) != length {
-		return nil, fmt.Errorf("%s is not length %d bytes", s, length)
+		return nil, fmt.Errorf("length of %s is not %d bytes", s, length)
 	}
 	return bytes, nil
 }