eth, les: add new config field SyncFromCheckpoint (#22123)

This PR introduces a new config field SyncFromCheckpoint for light client.

In some special scenarios, it's required to start synchronization from some
arbitrary checkpoint or even from the scratch. So this PR offers this
flexibility to users so that the synchronization start point can be configured.

There are two relevant configs: SyncFromCheckpoint and Checkpoint.

- If the SyncFromCheckpoint is true, the light client will try to sync from the
  specified checkpoint.

- If the Checkpoint is not configured, then the light client will sync from the
  scratch(from the latest header if the database is not empty)

Additional notes: these two configs are not visible in the CLI flags but only
accessable in the config file.

Example Usage:

[Eth]
SyncFromCheckpoint = true

[Eth.Checkpoint]
SectionIndex = 100
SectionHead = "0xabc"
CHTRoot = "0xabc"
BloomRoot = "0xabc"

PS. Historical checkpoint can be retrieved from the synced full node or light
client via les_getCheckpoint API.

les/client_handler.go

@@ -44,9 +44,12 @@ type clientHandler struct {
 	downloader *downloader.Downloader
 	backend    *LightEthereum
 
-	closeCh  chan struct{}
-	wg       sync.WaitGroup // WaitGroup used to track all connected peers.
-	syncDone func()         // Test hooks when syncing is done.
+	closeCh chan struct{}
+	wg      sync.WaitGroup // WaitGroup used to track all connected peers.
+
+	// Hooks used in the testing
+	syncStart func(header *types.Header) // Hook called when the syncing is started
+	syncEnd   func(header *types.Header) // Hook called when the syncing is done
 }
 
 func newClientHandler(ulcServers []string, ulcFraction int, checkpoint *params.TrustedCheckpoint, backend *LightEthereum) *clientHandler {