feat: add keycloak plugin for better SSO UX

[rocketeerbkw]

General Checklist

• Affected Issues have been mentioned in the Closing issues section
• Documentation has been written/updated
• PR title is ready for inclusion in changelog

Database Migrations

• If your PR contains a database migation, it MUST be the latest in date order alphabetically

Description

Adds a keycloak plugin that can detect which identity provider to use based on the user email address, instead of forcing the user to select their own provider.

Configuration of the login flow is manual for this PR, there is an example at https://sventorben.github.io/keycloak-home-idp-discovery/configuration.html#authenticator-execution.

For local testing, you can run make compose/example-sso and then create a new login flow for the lagoon realm. Note that the lagoon theme is quite out of date and doesn't work well with the new flow.

Closing issues

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