feat: implement fieldMapping to OnePasswordItem defintion

[jke-tapesandpips]

✨ Summary

Adding a fieldMapping to have custom Fields in the Kubernetes Secret, but not in the 1Password Item

---
apiVersion: onepassword.com/v1
kind: OnePasswordItem
metadata:
  name: harbor-postgres-credentials
  namespace: op-connect
spec:
  itemPath: "vaults/vaultname/items/itemname"
  fieldMapping:
    password: myCustomField
    username: myCustomUsernameField

🔗 Resolves:

#273

✅ Checklist

• 🖊️ Commits are signed
• 🧪 Tests added/updated: (See the Testing Guide for when to use each type and how to run them)
  • 🔹 Unit
  • 🔸 Integration
  • 🌐 E2E (Connect) (not needed, since only the Kubernetes Secret Logic changed)
  • 🔑 E2E (Service Account) (not needed, since only the Kubernetes Secret Logic changed)
• 📚 Docs updated (if behavior changed)

🕵️ Review Notes & ⚠️ Risks