[FR]: Firebase Remote Config should support applications to set device country/region

### Description

1. Testing ability for different countries
2. Currently device region/country can only be detected using the IP address of the device.
3. SDK should give us the ability to set region or it can get active region on device settings by using `Locale.current.regionCode`

According to the [Firebase Remote Config manager docs](https://firebase.google.com/docs/remote-config/parameters?hl=en&authuser=0), Firebase Remote Config determines user country/region by device IP address. This makes the test process hard because we may want to test UI in different countries. I read the documents and investigated the open-source codes. As a result, I couldn't find a way to set the country/region inside the application. I think we should have an SDK API to set the device country/region by app. In such a case we can use `Locale.current.regionCode` to set the device region and test different configurations in different countries easily.

### API Proposal

_No response_

### Firebase Product(s)

Remote Config

Start by reviewing the Firebase iOS SDK's Remote Config implementation and the linked Firebase Remote Config manager documentation. No source file, test, or API design is named; done would require an agreed SDK API that lets applications provide or use the device country/region for testing Remote Config configurations.

Written by the indexing model from the issue text.