Default backend V2 url on Dev builds to STG

[CassioMG]

What

Default backend V2 to STG url for Freighter Dev builds (instead of using PROD url).

We should do the same for backend V1 as soon as STG is publicly available there.

This PR also renames the backend config file and types/functions to make it more explicit that the backend env is only configurable on Dev builds.

Why

So we prevent Dev builds from polluting PROD backend with Dev requests.

Known limitations

N/A

Checklist

PR structure

• This PR does not mix refactoring changes with feature changes (break it down into smaller PRs if not).
• This PR has reasonably narrow scope (break it down into smaller PRs if not).
• This PR includes relevant before and after screenshots/videos highlighting these changes.
• I took the time to review my own PR.

Testing

• These changes have been tested and confirmed to work as intended on Android.
• These changes have been tested and confirmed to work as intended on iOS.
• These changes have been tested and confirmed to work as intended on small iOS screens.
• These changes have been tested and confirmed to work as intended on small Android screens.
• I have tried to break these changes while extensively testing them.
• This PR adds tests for the new functionality or fixes.

Release

• This is not a breaking change.
• This PR updates existing JSDocs when applicable.
• This PR adds JSDocs to new functionalities.
• I've checked with the product team if we should add metrics to these changes.
• I've shared relevant before and after screenshots/videos highlighting these changes with the design team and they've approved the changes.

[CassioMG]

this is there we set the default values