UI Settings should have a getWithInternal method

[tsullivan]

Currently, all the methods in src/ui/settings/index.js require a req object to be passed in. There are times when we need to get or set a uiSetting outside of a request, for example if we need to set it to a default at startup time for a plugin, or when we need a background task to read a setting value.

It seems a req parameter is required because the getUserProvided method uses callWithRequest internally: https://github.com/elastic/kibana/blob/v6.0.0-alpha1/src/ui/settings/index.js#L82

There could be additional methods added to src/ui/settings that allow querying ES with callWithInternalUser that would work for reading UI Settings at plugin startup time, or in background tasks.

cc @spalger @kjbekkelund @azasypkin

[spalger]

if we need to set it to a default at startup time for a plugin

I would hope that this doesn't require writing to elasticsearch

I agree that there should be some way to get the uiSettings out of elasticsearch without a request, but we should be very careful about this.

I've grown rather fond of a pattern we used in the SavedObjectClient, were a partially applied version of callWithRequest is passed in that can be used to make whatever requests are neccessary. Using this pattern we could update uiSettings to read from elasticsearch without having to teach it about the clusters, call with request, or anything like that.

[tsullivan]

if we need to set it to a default at startup time for a plugin

If we have #10863, and could define settings in the uiExport of a plugin, then this isn't a concern