forge: add forgejo sources

basic/infrastructure/kustomizations/forgejo-kustomization.yaml

+apiVersion: kustomize.toolkit.fluxcd.io/v1beta2
+kind: Kustomization
+metadata:
+  name: forgejo
+  namespace: flux-system
+spec:
+  interval: 10m
+  retryInterval: 1m
+  wait: true
+  timeout: 3m
+  dependsOn:
+    - name: single-sign-on
+  sourceRef:
+    kind: GitRepository
+    name: stackspout
+  path: ./basic/apps/forge
+  prune: true
+  postBuild:
+    substituteFrom:
+      #- kind: Secret
+      #  name: stackspin-forgejo-variables
+      - kind: Secret
+        name: stackspin-forgejo-oauth-variables
+      - kind: Secret
+        name: stackspin-cluster-variables

basic/infrastructure/sources/forgejo.yaml

+apiVersion: source.toolkit.fluxcd.io/v1beta1
+kind: HelmRepository
+metadata:
+  name: forgejo
+  namespace: flux-system
+spec:
+  interval: 60m
+  url: oci://codeberg.org/forgejo-contrib/forgejo