Web OAuth Support

Is your feature request related to a problem? Please describe.
Currently, the iOS app allows users to log in with a user account that exists on the LMS, or using a handful of social media sources. However, many instances of the platform use unique authentication providers which would have to be explicitly built under the current methodology.

It is not practical to build a special button for every possible authentication provider. For example, an instance may have SAML configured for their SSO, and may require that all users authenticate to it.

Describe the solution you'd like
The platform already provides a generic method of authenticating to it via OAuth. This is required for the MFEs to work. The actual user authentication may be deferred to an upstream provider, but the platform itself provides an OAuth endpoint handshake that is used for any auxiliary service. The mobile apps can take advantage of this to allow for generic 'web view authentication'.

Using web view authentication is slightly less smooth than having a built-in supported authentication form. However, it is universally functional, and can work with any instance in any authentication configuration without the need for constructing additional UI elements once this feature is implemented.

Describe alternatives you've considered
Building a separate UI/button for each authentication provider is possible but requires each team with their own authentication method to keep track of significant changes and continually rebase them.

Additional context
This change would also be useful for the Android app.

Review the existing iOS login flows and how the platform's generic OAuth endpoint handshake is used. Define the web-view authentication flow and verify that it supports instances with different upstream providers; the issue does not name specific files or tests, so those must be located first.