Point in time based searches should provide an option to ignore missing indices

[SingingTree]

Is your feature request related to a problem? Please describe.
If a point in time is created against an index that is then deleted, searches using that point in time will fail. This is problematic if a point in time is created against all indices, and one of those indices is then dropped while the PIT is still in use. An example of this occurring is that lifecycle management may drop an old index in the middle of a search.

A minimal way to reproduce this is:

1. Stand up an OpenSearch cluster.
2. Add 2 indices -- index-1 and index-2
3. Create a PIT against _all
4. (Optionally) Test that a search with the PIT works
5. Delete index-2
6. Test that a search with the PIT fails with a 404 indicating that an index is missing.

Describe the solution you'd like
Allow PITs to specify something like allow_partial_search_results which then returns search results while ignoring missing indices similar to non-PIT searches.

Describe alternatives you've considered
This can be managed on the client by creating a new PIT and searching again, however this is cumbersome.

Additional context
Elasticsearch have tracked a similar issue in elastic/elasticsearch#81256 and have implemented a fix to the problem described in elastic/elasticsearch#81349

[msfroh]

As an admin task, we should probably create a dedicated label for point-in-time search, to ensure that the right folks get their eyes on this. Those of us who follow the Search label probably don't have enough context to dive in on PIT stuff.

In the meantime, @bharath-techie -- do you know of a project where we can assign this so that it gets some traction? Thanks!

[bharath-techie]

The one in which we were tracking PIT backend tasks is closed - #1147

We can create a new one if needed.