Allow to add clusters whose names already exist in the list

[joaquimrocha]

When loading clusters from a file, it will detect if we have clusters with that name configured and tell the user to edit the name before adding them, but this is not a great UX.

I think the solution for this could be:

1. Suffix solution: add a suffix to the clashing names and suggest to the user that they can add those clusters with the proposed new suffix;
2. User choice solution: allow the user to edit the name before they are allowed to actually load the cluster
3. Allow name clashes from different origins: its unlilkely that the user wants to add the same exact cluster/context twice. But they may have the same cluster name in different config files. We could support this by including the notion of origin when referring to clusters both in backend and frontend. This would be about encoding the origin (kubeconfig file path, public cloud name+subscription, etc.) somehow.

We need to decide on one or more ways forward.