Refactor: make pluginProps unique to each plugin

Currently, we have a way to pass a pluginProps attribute to the PluginSlot component. This will then pass those props to each Plugin.

While there is value in being able to apply the same props to each Plugin, there are times when we would want to pass unique props to each one. For example, being able to pass a specific class or style to a specific plugin is something we would like to support.

This can probably be accomplished by adding these unique props to the config for that plugin. Currently, however, our PluginContainers (such as PluginContainerIframe) do not account for any additional props being in the config.

We’d like to make sure the current behavior is unaffected — we still want to be able to pass “universal” props to every Plugin.

Acceptance Criteria:

• Provide a way to pass unique props to each plugin
• Original functionality is unaffected
• Document how to use these props in README and wiki page

Start by reading the PluginSlot component and src/plugins/PluginContainerIframe.jsx to trace how universal pluginProps and per-plugin config are passed to each Plugin. Review the other PluginContainers for the same limitation, then update the README and wiki with the usage. Done means unique props work per plugin while universal props still apply to all plugins.