With cross cluster search today, each cluster has to be specified separately, eg

GET one:*,two:*,three:*/_search

It would be good to be able to support wildcards:

GET *:*/_search

By extension, (because it is how it works everywhere else), we should support pattern matching like:

GET t*:*/_search

One complication is that : is an allowed character in index names today (although we have plans to deprecate #23892). We can work around this with the following logic:

• if remote clusters are not configured, treat the specified name as an index name
• if remote clusters ARE configured, treat everything before the first : as a cluster name