Destination Google Cloud Storage (GCS) : testing against AWS S3

### Connector Name

destination-gcs

### Connector Version

0.3.0

### What step the error happened?

Configuring a new connector

### Revelant information

Tried to setup a destination for GCS, added all the info (bucket, path, HMAC key, secret) and when I click on "set up destination" it shows me the following error:

```
Configuration check failed

State code: AccessDenied; Message: Access denied. (Service: Amazon S3; Status Code: 403; Error Code: AccessDenied; Request ID: null; S3 Extended Request ID: null; Proxy: null)
```

It seems it's trying to verify the GCS credentials against AWS S3 instead of GCS.

### Relevant log output

```shell
Internal message: com.amazonaws.services.s3.model.AmazonS3Exception: Access denied. (Service: Amazon S3; Status Code: 403; Error Code: AccessDenied; Request ID: null; S3 Extended Request ID: null; Proxy: null), S3 Extended Request ID: null
Failure origin: destination
Failure type: config_error
```

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