Add light sync command

- Only syncs block headers (excludes block bodies, transactions, receipts, and logs)
- Modifies validation window to include the most recent block
- Isolates validation window to the variable defined in the cmd directory (blocks
  have a separate variable defined in the block_repository for determining when
  to set a block as final)

pkg/history/header_validator.go

@@ -0,0 +1,27 @@
+package history
+
+import (
+	"github.com/vulcanize/vulcanizedb/pkg/core"
+	"github.com/vulcanize/vulcanizedb/pkg/datastore"
+)
+
+type HeaderValidator struct {
+	blockChain       core.Blockchain
+	headerRepository datastore.HeaderRepository
+	windowSize       int
+}
+
+func NewHeaderValidator(blockChain core.Blockchain, repository datastore.HeaderRepository, windowSize int) HeaderValidator {
+	return HeaderValidator{
+		blockChain:       blockChain,
+		headerRepository: repository,
+		windowSize:       windowSize,
+	}
+}
+
+func (validator HeaderValidator) ValidateHeaders() ValidationWindow {
+	window := MakeValidationWindow(validator.blockChain, validator.windowSize)
+	blockNumbers := MakeRange(window.LowerBound, window.UpperBound)
+	RetrieveAndUpdateHeaders(validator.blockChain, validator.headerRepository, blockNumbers)
+	return window
+}