[FEATURE] Allow Transform job to target alias index

[dragosrosculete]

Right now Transform can only target an existing index or create a new one. It does not work if an alias index is provided.
This makes it impossible to automate the process and integrate with rollover jobs .

More details:
Lets take the following indexes as example, created hourly

elb-metrics-2022-12-19-01;
elb-metrics-2022-12-19-02;
elb-metrics-2022-12-19-03 ; etc ..

From transform job we can select as a source a wildcard of indexes, for example elb-metrics-* ( this will include present and future indexes). But on the target index I only have the option to select an existing index, for example transformed-metrics or it will create this one if it doesn’t already exist .
Now this transform jobs runs continuously, so once it is set up, it will write from source (wildcard indexes) to this transformed-metrics index.
The problem is that this index transformed-metrics will eventually have months to years of data but we only want to keep it for a few months.
And there is no way to rotate or create transformed-metrics-2 because the continuous job is set to write into transformed-metrics.
I want to be able to write in a target alias if possible so I can then rotate these indexes and keep only some.

I tried to have Transform to a write into an alias index but this doesn’t work, it does not support to target an alias
Error: Failed to index the documents

[bowenlan-amzn]

Similar request for rollup
#61
#445

[bowenlan-amzn]

#226 is related and best to be resolved along with this one

[n-dohrmann]

Could I be assigned to this?