Batch outspends requests

fixes #1902

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

@@ -13,6 +13,7 @@ export interface AbstractBitcoinApi {
   $getAddressPrefix(prefix: string): string[];
   $sendRawTransaction(rawTransaction: string): Promise<string>;
   $getOutspends(txId: string): Promise<IEsploraApi.Outspend[]>;
+  $getBatchedOutspends(txId: string[]): Promise<IEsploraApi.Outspend[][]>;
 }
 export interface BitcoinRpcCredentials {
   host: string;