This change adds a createComponents() method to Plugin implementations
which they can use to return already constructed componenents/services.
Eventually this should be just services ("components" don't really do
anything), but for now it allows any object so that preconstructed
instances by plugins can still be bound to guice. Over time we should
add basic services as arguments to this method, but for now I have left
it empty so as to not presume what is a necessary service.
The only reason for LifecycleComponent taking a generic type was so that
it could return that type on its start and stop methods. However, this
chaining has no practical necessity. Instead, start and stop can be
void, and a whole bunch of confusing generics disappear.
As discussed at https://github.com/elastic/elasticsearch-cloud-azure/issues/91#issuecomment-229113595, we know that the current `discovery-azure` plugin only works with Azure Classic VMs / Services (which is somehow Legacy now).
The proposal here is to rename `discovery-azure` to `discovery-azure-classic` in case some users are using it.
And deprecate it for 5.0.
Closes#19144.
Ryan Ernst
David Pilato