Backport of #53982
In order to prepare the `AliasOrIndex` abstraction for the introduction of data streams,
the abstraction needs to be made more flexible, because currently it really can be only
an alias or an index.
* Renamed `AliasOrIndex` to `IndexAbstraction`.
* Introduced a `IndexAbstraction.Type` enum to indicate what a `IndexAbstraction` instance is.
* Replaced the `isAlias()` method that returns a boolean with the `getType()` method that returns the new Type enum.
* Moved `getWriteIndex()` up from the `IndexAbstraction.Alias` to the `IndexAbstraction` interface.
* Moved `getAliasName()` up from the `IndexAbstraction.Alias` to the `IndexAbstraction` interface and renamed it to `getName()`.
* Removed unnecessary casting to `IndexAbstraction.Alias` by just checking the `getType()` method.
Relates to #53100
4b4fbc160d