This commit adds a module emitting a deprecation warning when a
dot-prefixed index is manually or automatically created, or when a
composable index template with an index pattern that uses a dot-prefix
is created. This pattern warns that in the future these indices will not
be allowed. In a future breaking change (10.0.0 maybe?) the deprecation
can then be changed to an exception.
These deprecations are only displayed when a non-operator user is using
the API (one that does not set the `X-elastic-product-origin` header).
