feat: add server-side pagination sort and search for centers component (#506)

- Add server-side pagination, sort and search for centers component
- Refactor code to meet current guidelines

See: #234

src/app/centers/centers.service.ts

@@ -1,17 +1,45 @@
+/** Angular Imports */
 import { Injectable } from '@angular/core';
 import { HttpClient, HttpParams } from '@angular/common/http';
+
+/** rxjs Imports */
 import { Observable } from 'rxjs';
 
+/**
+ * Clients service.
+ */
 @Injectable({
     providedIn: 'root'
-  })
-
-
+})
 export class CentersService {
-    constructor(private http: HttpClient) { }
+  /**
+   * @param {HttpClient} http Http Client to send requests.
+   */
+  constructor(private http: HttpClient) { }
 
-    getCenters(): Observable<any> {
-        return  this.http.get('/centers');
-    }
+  /**
+   * @param {any} filterBy Properties by which entries should be filtered.
+   * @param {string} orderBy Property by which entries should be sorted.
+   * @param {string} sortOrder Sort order: ascending or descending.
+   * @param {number} offset Page offset.
+   * @param {number} limit Number of entries within the page.
+   * @returns {Observable<any>} Centers.
+   */
+  getCenters(filterBy: any, orderBy: string, sortOrder: string, offset: number, limit: number): Observable<any> {
+    let httpParams = new HttpParams()
+      .set('offset', offset.toString())
+      .set('limit', limit.toString())
+      .set('sortOrder', sortOrder)
+      .set('orderBy', orderBy)
+      .set('paged', 'true');
+    // filterBy: name, externalId
+    filterBy.forEach(function (filter: any) {
+      if (filter.value) {
+        console.log(filter.value);
+        httpParams = httpParams.set(filter.type, filter.value);
+      }
+    });
+    return this.http.get('/centers', { params: httpParams });
+  }
 
 }