Advanced settings toggle for new EUI theme

[snide]

Summary

EUI is producing a new theme that includes a bunch of stylistic upgrades. As it progresses, we'll need a toggle within Kibana to switch between the themes so we can see how those changes cascade into the product. This will also let users play with this change and give feedback before we fully commit to them as a default.

EUI currently spits out dist files for the new css and json under the amsterdam name schema.

Specifics

• This should probably just be a string enum in case we have other themes
• Any instances where eui_theme_xxx.css is imported will need to be made dynamic
• Any instances where eui_theme_xxx.json is imported will need to be made dynamic
• The _styling_constants.scss file will need to dynamically pull from the new theme now based upon this toggle.
• The KUI sass files will need to have compiled css for the new theme
• Kibana will now need to shift to using the newer version of the Inter UI font, which the new theme utilizes.

@cchaos and I have familiarity with a lot of these systems, as does @spalger and we can likely point you towards the correct files.

[elasticmachine]

Pinging @elastic/kibana-core-ui (Team:Core UI)

[spalger]

I'm happy to discuss strategy around, the build system can be used for a certain amount of this.