[EuiProvider] App configuration provider

[thompsongl]

Expand the use of the current EuiProvider layer beyond i18n, or create a coordinated system of context layers that allow for global or application-level settings for various options in EUI.

Current ideas:

• Theming, as introduced in [Meta][CSS-in-JS] Switching to emotion #3912
• EuiPopover, such as buffer
• a11y, such as enforcing stricter standards in certain applications
• domains, as used in getSecureRelForTarget
• Link components, such as replacing the underlying element used in EuiLink, etc.
• Feature flags
• Passing of z-index or other props based on their parent being in a fixed position component (like [EuiFlyout] Popover and other positioned elements within should not scroll with the page #3456)

[thompsongl]

#5121 will add EuiProvider, and EuiContext will move to be renamed Eui18nProvider, as discussed.