Fold plugin script into elasticsearch script with -plugin option

[kimchy]

Today, we highjack a generic script name called plugin to install plugins with ES. Other products might have this script name and it would be nice if we are better citizens of $PATH here. We should allow to install plugins using the elasticsearch script, and deprecate the plugin script.

[jimmyjones2]

Same as #11651?

[proegssilb]

Fixes the same problem as #11651, but different proposal. This approach would still make packaging on Linux a lot easier, unless a distro decides to replace the elasticsearch script (I don't work on distros, I don't know how likely it is).

[clintongormley]

We've decided against doing this as bin/elasticsearch should be run as the elasticsearch user, and bin/plugin as root

[jimmyjones2]

This is fine, but a bit unfortunate that downstream packagers will always have to use a different name to upstream. How about elasticsearch-plugin?

[Hubbitus]

In Fedora we already use name elasticsearch-plugin. But in any case as it should be compiled from source without network build procedure and start script significant differs than shipped upstream. Meantime it will be helpful if such distinguished name will appear in docs.

[clintongormley]

@Hubbitus this is happening in #16022

[Hubbitus]

@clintongormley thank you.