[Fleet] Add attribute allowNoIndex to Fleet index patterns

[neptunian]

This #82223 changes how index patterns are loaded into the UI by looking in ES to create the index patterns. In the case of Fleet, indices may not exist yet. Because of our index placeholder which has no fields, it would overwrite the index patterns in our saved objects (logs-*, metrics-*) to nothing. You can see this issue by going to Fleet app (new index patterns will be created with the installed packages fields, such as logs-*). Go to Stack Management -> Index Patterns -> logs-*. There are no fields in the index patterns.

To solve this #85370 will continue to use the saved object index patterns and if we add a new attribute ignoreIndexNotFound, it will not 404 when it finds no indices. No error message will be displayed.

For Fleet this means we need to make a few changes:

• remove the index placeholders we create and check for during setup
• remove readFromDocValues in each field
• add the allowNoIndex attribute to our index patterns during creation
• create a migration to add the above attribute to the existing index patterns

Related: #65284
Depends on: #85370

[elasticmachine]

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

[neptunian]

Changes for this will be in this PR #85370.

The readFromDocValues will be done in a separate PR after FF as it is not doing anything right now and i'd rather lower the risk of changing that.

@mattkime Will be doing the index-pattern saved object migration as the migration should be written in the data plugin.

[neptunian]

Most changes made in #85370