Closes#20640.
This PR introduces a new parameter to v2 templates, `allow_auto_create`,
which allows templates to override the cluster setting `auto_create_index`.
Notes:
* `AutoCreateIndex` now looks for a matching v2 template, and if its
`allow_auto_create` setting is true, it overrides the usual logic.
* `TransportBulkAction` previously used `AutoCreateIndex` to check
whether missing indices should be created. We now rely on
`AutoCreateAction`, which was already differentiating between creating
indices and creating data streams. I've updated `AutoCreateAction` to
use `AutoCreateIndex`. Data streams are also influenced by
`allow_auto_create`, in that their default auto-create behaviour can
be disabled with this setting.
* Most of the Java file changes are due to introducing an extra
constructor parameter to `ComposableIndexTemplate`.
* I've added the new setting to various x-pack templates
* I added a YAML test to check that watches can be created even when
`auto_create_index` is `false`.
James Rodewig
Rory Hunter
debadair