[Fleet] Use index pattern api to create/update index patterns

[neptunian]

We should use the new index pattern api to create and update index patterns in Fleet during package installation/upgrade/removal. Before using it, support will need to be added to handle our use case of being able to pass the option "allowNoIndex" in order to prevent the field caps api returning 404 when no indices exist to match an index pattern yet.

We currently write to index pattern SOs directly in https://github.com/elastic/kibana/blob/master/x-pack/plugins/fleet/server/services/epm/kibana/index_pattern/install.ts#L134-L139

[elasticmachine]

Pinging @elastic/ingest-management (Team:Ingest Management)

[jen-huang]

@ph Thanks for finding this issue, I've linked it to the larger Kibana meta.

This effort is currently blocked as the new index pattern service does not support defining non-scripted fields to be saved, which is what EPM does. Discussions on actively happening on related index pattern issues to find a solution for this. More code details in #94490.

[joshdover]

@hop-dev Do you think this issue is still relevant? I wonder if we should close it now that we're using import/export for index pattern creation?

[hop-dev]

@joshdover I believe it could be still relevant yes, using the API seems more robust that creating the saved objects directly. We currently import the index templates every time we import saved objects to avoid the reference checking bug at the time, I believe this issue has since been solved here so we could re-visit this.

We no longer set fields on the index template so I believe we no longer need the flag that was created for us above 😅

[jen-huang]

Closing in favor of #153368.