Improve 1st party app login exp without popup browser

**Problem**
It's quite common for dev to use Authgear as 1st party app login (in-contrary to 3rd parties); And many think popup a browser with a cancel button is really bad for UX.

This is a pitch to explore alternatives -- e.g. how can we make it very easy to embed the login/signup page (or as an native view controller?) instead of popup (e.g. `ASWebAuthenticationSession`) when the dev is using it as 1st party login (hence no security consideration?) and do not need the device single sign-on functionality.

**Appetite**
4 - 6 weeks

**Solution**
It requires some technical research, including:
1. Security implication
2. The SDK API interfaces on various platform.

**Rabbit holes**
Details about the solution worth calling out to avoid problems

**No-goes**
Functionality or use cases we intentionally aren’t covering to fit the appetite

No files or tests are named. Start by researching the security implications of avoiding ASWebAuthenticationSession and comparing SDK API interfaces across platforms. Done means documenting a viable embedded or native-view login alternative for first-party apps, including its security trade-offs and scope.

Written by the indexing model from the issue text.