Add difficulty adjustment endpoints methods.

src/app/bitcoin/difficulty.ts

@@ -0,0 +1,16 @@
+import { AxiosInstance } from 'axios';
+import {
+  Adjustment,
+  DifficultyInstance,
+} from '../../interfaces/bitcoin/difficulty';
+
+export const useDifficulty = (api: AxiosInstance): DifficultyInstance => {
+  const getDifficultyAdjustment = async () => {
+    const { data } = await api.get<Adjustment>(`/difficulty-adjustment`);
+    return data;
+  };
+
+  return {
+    getDifficultyAdjustment,
+  };
+};