Add keycloak

Keycloak is a login provider that we can host to give us SSO. This is preferable
to all of our services having different authentication capabilities, logins etc.
(e.g. mailu doesn't support 2fa: <https://github.com/Mailu/Mailu/issues/2222>!)

Change-Id: Ic0a5238a03d4d0b8a270c29a270c579b00aea799
diff --git a/flake.nix b/flake.nix
index 7d0d732..838808d 100644
--- a/flake.nix
+++ b/flake.nix
@@ -61,6 +61,7 @@
               ./modules/git.nix
               ./modules/grafana.nix
               ./modules/home-manager-users.nix
+              ./modules/keycloak.nix
               ./modules/kitty.nix
               ./modules/loginctl-linger.nix
               ./modules/matrix.nix