Decouple auth from Frontegg

[ZIJ]

Currently the only OAuth provider supported by Digger backend is Frontegg. This is for historical reasons from back when we were just launching Digger Cloud. The main requirement was quick setup and user-facing admin portal so that users could manage their tokens and users themselves. Frontegg (unlike Auth0 and other "simple" OAuth providers) did offer that extra layer of user-facing user management.

There are alternatives to Frontegg though, including some open source ones. Most importantly, it doesn't make sense for an open source product to depend on a non-open-source auth provider. Users should be able to choose which auth to use; we need to cleanly decouple from an auth provider. Also Enterprise Edition features like SSO / SAML require it to be a separate concern.