Allow KibanaThemeProvider to be used in preboot plugins

[jportner]

While implementing #119124, I realized that the interactive_setup plugin cannot have a dependency on the kibana_react plugin, because the former is a "preboot" plugin and the latter is a "standard" plugin.

I decided to copy the KibanaThemeProvider code to the interactive_setup plugin as a short-term solution. However, we should come up with a way to remove this duplication in the future.

At a high level I can see a couple of viable options:

1. Move the KibanaThemeProvider to a package (e.g., @kbn/theme-provider).
2. Change the PluginsService to allow a "preboot" plugin to require a "standard" plugin's bundle, and change the Server, RenderingService, and bootstrapRendererFactory to provide a subset of "standard" bundles while in "preboot" mode. This sounds more complex, but it might be better long-term as we eventually add more "preboot" plugins and add more dependencies from other places in Kibana.

More context is in my comment here: #119124 (comment)

[elasticmachine]

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