Added missing block header API (#630)

* header API frontend

* Block Header API endpoint added to Node.js backend

* updated package-lock.json

Co-authored-by: Rishabh <rishabh@Rajeshs-MacBook-Pro.local>

backend/src/api/bitcoin/bitcoin-api-abstract-factory.ts

@@ -6,6 +6,7 @@ export interface AbstractBitcoinApi {
   $getBlockHeightTip(): Promise<number>;
   $getTxIdsForBlock(hash: string): Promise<string[]>;
   $getBlockHash(height: number): Promise<string>;
+  $getBlockHeader(hash: string): Promise<string>;
   $getBlock(hash: string): Promise<IEsploraApi.Block>;
   $getAddress(address: string): Promise<IEsploraApi.Address>;
   $getAddressTransactions(address: string, lastSeenTxId: string): Promise<IEsploraApi.Transaction[]>;