…c#120797)

This class is confusing: - It contains an **unused** `cluster` attribute
- we never separate out the cluster, it remains in the `index` field.
Also, in the constructor, this field is called `catalog`, which is a
concept entirely absent from ESQL at the moment. - It can refer to
multiple indices, even multiple wildcard patterns, but doesn't mention
this neither in its name nor javadoc. - It has little to do with tables,
which is likely a remnant of this class' usage in SQL, before the
`esql.core` split.

This PR removes the `cluster` attribute, renames the class to
`IndexPattern`, and adds javadoc to clarify that it can also contain
stuff like `remote1:idx1,remote-*:idx-*`.

(cherry picked from commit 060c833)