Add Mongo Proxy to Data Explorer (Standalone and Portal) (#1738)

* Mongo Proxy backend API

* merge main into current

* allow mongo proxy endpoints to be constants

* allow mongo proxy endpoints to be constants

* fix test

* check for allowed mongo proxy endpoint

* check for allowed mongo proxy endpoint

---------

Co-authored-by: Asier Isayas <aisayas@microsoft.com>

src/Common/Constants.ts

@@ -211,6 +211,10 @@ export class HttpHeaders {
   public static migrateOfferToAutopilot: string = "x-ms-cosmos-migrate-offer-to-autopilot";
 }
 
+export class ContentType {
+  public static applicationJson: string = "application/json";
+}
+
 export class ApiType {
   // Mapped to hexadecimal values in the backend
   public static readonly MongoDB: number = 1;