fix: generate endpoint to service with '/api_v3/' as a prefix.

BREAKING CHANGE:

before:
The service api provided by the application included `/api_v3/` when provided
```
"https://www.kaltura.com/api_v3/"
```

after:
The service api provided by the application shouldn't include `/api_v3/` when provided
```
"https://www.kaltura.com"
```

src/kaltura-clients/kaltura-http-client-base.ts

@@ -217,7 +217,7 @@ export abstract class KalturaHttpClientBase extends KalturaClientBase {
     protected abstract _createCancelableAction(data: { endpoint: string, headers: any, body: {} }): CancelableAction;
 
     private _createEndpoint(parameters: any): string {
-        let endpoint = `${this.endpointUrl}/service/${parameters.service}`;
+        let endpoint = `${this.endpointUrl}/api_v3/service/${parameters.service}`;
         if (parameters.action) {
             endpoint = `${endpoint}/action/${parameters.action}`;
         }