App-to-App (App2App) Authentication

## Problem

Allow authenticating through another app under different publisher, which is installed in the same device.
For example, in open-banking applications, granting Bank A app access to Bank B app to allow info exchange or authorize a transfer.

## Appetite

4 weeks

## Solution

The design is specified in:
[https://github.com/authgear/authgear-server/blob/main/docs/specs/app2app.md]()

## Todo

- [X] Server side support
- [X] Project Config support
- [ ] SDK update
- [X] Android SDK
- [X] iOS SDK
- [ ] JS SDK
- [ ] Flutter SDK
- [ ] Xamarin SDK
- [X] Allow configuration in the Portal