ci: add action for STACKIT login

.github/actions/login_stackit/action.yml

@@ -0,0 +1,16 @@
+name: STACKIT login
+description: "Login to STACKIT"
+inputs:
+  serviceAccountToken:
+    description: "Credentials authorized to create Constellation on STACKIT."
+    required: true
+runs:
+  using: "composite"
+  steps:
+   - name: Login to STACKIT
+     env:
+       UAT: ${{ inputs.serviceAccountToken }}
+     shell: bash
+     run: |
+        mkdir -p ~/.stackit
+        echo "${UAT}" > ~/.stackit/credentials.json