[build] Add data directory for plugins

[jbudz]

Closes #7157, also part of #7275.

This adds a data directory that plugins can write files to. For archives, this live at installedPlugins/.data, and for packages at /var/lib/kibana.

The data folder path is added as part of the config object, config.get('path.data')

[w33ble]

Nice! Are there any plans to expose the .data path programmatically, maybe via the kibana or server object? It would be nice if we could just tell plugin authors to use a service of some kind to control where they write data, instead expecting them to handle the paths themselves.

[jbudz]

Good call, it definitely should. I'm not quite sure on implementation yet but it's on the list.

[w33ble]

it's on the list.

Is there an issue you can cross-link?

[jbudz]

Checkbox one of #7275

[jbudz]

@w33ble gave it a go adding it to our config object. Names match https://www.elastic.co/guide/en/elasticsearch/reference/master/rpm.html#rpm-layout

[jbudz]

this should probably be removed, at this point it's already loaded

[w33ble]

Updated reporting and used the setting, everything worked as expected.

The only feedback I have is the use of config.get('path.data'). That's a pretty generic namespace, but the target path (installedPlugins/.data) is clearly meant for "external" plugins. Even as I was typing this, I was thinking that the distinction between "internal" (packaged by default) and "external" plugins was odd.

I know the package changes already went in, so I'm OK leaving this alone for now, but for the next release, it might be better to move .data to the root path. At the very least I'd like to see a discussion issue opened for this, if not an actual issue about moving .data.

Other than that, this LGTM!

[jbudz]

Opened #7519