feat(core): Add a Generic/General OAuth client

[kprinssu]

This PR contains changes in order to add a generalised OAuth client.

Please note that I am aware this feature is behind a pay-wall and I absolutely respect the Tabby team's decision to keep this behind a pay-wall. I want to emphasise this is a POC and my intent here was to quickly learn Rust and to give back to Tabby.

The benefit of having a generalised OAuth client is that end users can integrate with 3rd party identity providers (such as Auth0, Okta, Authentik, etc.). This is achieved by using OpenID Connect Discovery which contains the endpoints required to:

• generate and start an authorization code flow
• validate and get access tokens
• grab user info

On the backend, I am introducing a new migration to add a config_url to the oauth_credential table. This column is a varchar(256) and should accomodate most identity provider's Connect Discovery endpoints,

I also changed the frontend here and here screenshots and explanations as to how the UX looks like:

Existing OAuth Integrations will not display the new Config URL text field

New Radio Button to show the General OAuth Configs:

View the show page for a created General OAuth Configs:

SSO Sign In Page:

Please also note this branch contains changes from #4323 and I am hoping to merge the upstream PR before merging this one in.

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[IamTaoChen]

Maybe, name it as OIDC

[kprinssu]

Maybe, name it as OIDC

I appreciate the feedback, however, I am going to wait for the Tabby maintainers to leave some feedback before I make any further changes.