* idxmgmt: create independent data streams Supporter

Create a completely separate Supporter when data
streams are enabled. If data streams are enabled,
ensure that no setup (template, ILM, pipeline)
related config is specified.

* beater: enabling data streams disables pipeline

If data streams are enabled, pipeline registration
is disabled and attempting to setup pipelines will
fail. No pipeline will be specified when indexing;
the pipeline must be specified in the index template.

Also, if the server is running managed (by Fleet),
the server will fail to start unless data streams
are enabled.

* systemtest: add tests for data streams

* idxmgmt: log warnings upon finding legacy config

If legacy config is defined along with data streams
config, log warnings rather than returning errors.