fix(core): User is allowed to create nonsensical clusters

# What happened?

There seems to be no validation either in the FE or the BE regarding creation of clusters. This allows users to create nonsensical clusters:

https://github.com/Aiven-Open/klaw/assets/20607294/72a00ecf-71d4-417e-85b3-c59fe5f2a616

However, the biger issue is that users are allowed to create clusters that *mostly* make sense, but will not work as intended because of a subtle typo or other hard to see issue.

# What did you expect to happen?

- Validation on the front end for superficial errors (typo, formatting ?)
- Validation on the backend for validity of the cluster that was added (tring to ping it, or a health check, or something?)

The FE fixes may be only implemented on the coral side (https://github.com/Aiven-Open/klaw/issues/2280), as we may not want to invest too much in the Angular app at this point.