UploaderStrategy still requires api_secret if parameter was already added externally

This issue affects all current http artifacts:
https://github.com/cloudinary/cloudinary_java/blob/090788607f1b92f895b7b79c67e1e628719666c6/cloudinary-http42/src/main/java/com/cloudinary/http42/UploaderStrategy.java#L45-L49

Compared to the implementation on cloudinary-android:

~~~java
if (requiresSigning(action, options)) {
String apiKey = ObjectUtils.asString(options.get("api_key"), this.cloudinary().config.apiKey);
if (apiKey == null)
throw new IllegalArgumentException("Must supply api_key");
if (options.containsKey("signature") && options.containsKey("timestamp")) {
params.put("timestamp", options.get("timestamp"));
params.put("signature", options.get("signature"));
params.put("api_key", apiKey);
} else {
String apiSecret = ObjectUtils.asString(options.get("api_secret"), this.cloudinary().config.apiSecret);
if (apiSecret == null)
throw new IllegalArgumentException("Must supply api_secret");
params.put("timestamp", Long.valueOf(System.currentTimeMillis() / 1000L).toString());
params.put("signature", this.cloudinary().apiSignRequest(params, apiSecret));
params.put("api_key", apiKey);
}
}
~~~

Cloudinary Android Source: https://github.com/cloudinary/cloudinary_android/blob/f7a0b32cfd9f2b6507bb6461043e6c89d9478c03/lib/src/main/java/com/cloudinary/android/UploaderStrategy.java#L43-L59

My suggestion is to implement that for each http artifact or simply refactor the android implementation upwards to the core `AbstractUploaderStrategy`.

This allows `cloudinary-java` users to not have to enter `api_secret` if say the signature is generated via other microservices/authorization servers. This also allows easier server-side integration tests.

While I understand the audience for `cloudinary-java` is more towards server users where the secret key is most likely exposed within a monolithic service, the two reasons above are currently forcing us to mangle a separate [UploaderStrategy](https://gist.github.com/amirulzin/e7aa38991b23a87f2ea82b06561b6bd3) which simply use the above Android part instead.