Add example usage for the X-chain block indexer API (ava-labs#2778)

Co-authored-by: Dan Laine <daniel.laine@avalabs.org>
Co-authored-by: Dhruba Basu <7675102+dhrubabasu@users.noreply.github.com>

Author: 3 people

indexer/examples/x-chain-blocks/main.go

@@ -0,0 +1,55 @@
+// Copyright (C) 2019-2022, Ava Labs, Inc. All rights reserved.
+// See the file LICENSE for licensing terms.
+
+package main
+
+import (
+	"context"
+	"fmt"
+	"log"
+	"time"
+
+	"github.com/ava-labs/avalanchego/indexer"
+	"github.com/ava-labs/avalanchego/vms/proposervm/block"
+	"github.com/ava-labs/avalanchego/wallet/chain/x"
+	"github.com/ava-labs/avalanchego/wallet/subnet/primary"
+)
+
+// This example program continuously polls for the next X-Chain block
+// and prints the ID of the block and its transactions.
+func main() {
+	var (
+		uri       = fmt.Sprintf("%s/ext/index/X/block", primary.LocalAPIURI)
+		client    = indexer.NewClient(uri)
+		ctx       = context.Background()
+		nextIndex uint64
+	)
+	for {
+		log.Printf("polling for next accepted block\n")
+		container, err := client.GetContainerByIndex(ctx, nextIndex)
+		if err != nil {
+			time.Sleep(time.Second)
+			continue
+		}
+
+		proposerVMBlock, err := block.Parse(container.Bytes)
+		if err != nil {
+			log.Fatalf("failed to parse proposervm block: %s\n", err)
+		}
+
+		avmBlockBytes := proposerVMBlock.Block()
+		avmBlock, err := x.Parser.ParseBlock(avmBlockBytes)
+		if err != nil {
+			log.Fatalf("failed to parse avm block: %s\n", err)
+		}
+
+		acceptedTxs := avmBlock.Txs()
+		log.Printf("accepted block %s with %d transactions\n", avmBlock.ID(), len(acceptedTxs))
+
+		for _, tx := range acceptedTxs {
+			log.Printf("accepted transaction %s\n", tx.ID())
+		}
+
+		nextIndex++
+	}
+}