Deprecate `:` in cluster and index names

[clintongormley]

We use : in cross cluster search to separate the cluster name from the index name, eg cluster:index. This can introduce ambiguity when you have an index named cluster:index (although this can be worked around by adding a cluster prefix: my_cluster:cluster:index.

Still, to avoid ambiguity, we should deprecate : in cluster and index names.

[glefloch]

I'm interested into working on this issue.
By deprecating : in cluster and index names, do you mean logging something at startup (or at index creation time) if the name contains :, or should we throw an exception to prevent adding it?

[jasontedor]

To be clear, the ambiguity is handled in the following way already today. If you try to search on cluster:index, there is a remote cluster aliased by cluster, and there is an index named cluster:index, then we fail the request and inform the user that they need to change the remote cluster alias to resolve the ambiguity.

[javanna]

good point @jasontedor . I also wonder if we need to add any validation to the cluster name like in #23911 or rather in cluster aliases used for cross cluster search, that don't necessarily have to match with the remote cluster names.

[javanna]

the other bit that comes to mind is that the same validation should be applied to both index names and aliases, something that we happened to forget about in the past.