Refactor the way that the console BE parses POST bodies (#2113)

This includes two changes:
1. Creating a base string-type adapter for use parsing to/from JSON
   classes that are represented as simple strings
2. Changing the object-provider methods so that the POST bodies should
   contain precisely the expected object(s) and nothing else. This way,
   it's easier for the frontend and backend to agree that, for instance,
   one POST endpoint might accept exactly a Registrar object, or a list
   of Contact objects.

console-webapp/src/app/shared/services/backend.service.ts

@@ -12,12 +12,13 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-import { Injectable } from '@angular/core';
 import { HttpClient, HttpErrorResponse } from '@angular/common/http';
-import { Observable, catchError, of } from 'rxjs';
-import { Contact } from '../../settings/contact/contact.service';
+import { Injectable } from '@angular/core';
+import { catchError, Observable, of } from 'rxjs';
 import { SecuritySettingsBackendModel } from 'src/app/settings/security/security.service';
 
+import { Contact } from '../../settings/contact/contact.service';
+
 @Injectable()
 export class BackendService {
   constructor(private http: HttpClient) {}
@@ -55,7 +56,7 @@ export class BackendService {
   ): Observable<Contact[]> {
     return this.http.post<Contact[]>(
       `/console-api/settings/contacts?registrarId=${registrarId}`,
-      { contacts }
+      contacts
     );
   }
 
@@ -85,7 +86,7 @@ export class BackendService {
   ): Observable<SecuritySettingsBackendModel> {
     return this.http.post<SecuritySettingsBackendModel>(
       `/console-api/settings/security?registrarId=${registrarId}`,
-      { registrar: securitySettings }
+      securitySettings
     );
   }
 }