Adding optional Blockstream esplora backend support.

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

@@ -0,0 +1,16 @@
+const config = require('../../../mempool-config.json');
+import { AbstractBitcoinApi } from './bitcoin-api-abstract-factory';
+import BitcoindApi from './bitcoind-api';
+import EsploraApi from './esplora-api';
+
+function factory(): AbstractBitcoinApi {
+  switch (config.BACKEND_API) {
+    case 'esplora':
+      return new EsploraApi();
+    case 'bitcoind':
+    default:
+      return new BitcoindApi();
+  }
+}
+
+export default factory();