Embeddability of condition and action forms basics

Goal

Let's plan how condition and action forms can be embedded by contrib modules & define the API necessary to do it, such that we can iterate and improve things in future without breaking that API.

Implementation concept

- Modules will have to define the available context for their condition / action component
- Modules create the component and use the API to generate, validate and submit the form of the component
- The embedded form will contain links for adding, editing or deleting the contained expressions via the regular Rules power UI.
- After submitting the form component, the module can export the configuration of the component and store it as part of its own config entity. Config entity dependencies etc. will have to be added based on information provided by the Rules component.
- For the contained links etc. to work, the module has to integrate routes provided by Rules. The routes must be able to fetch the edited Rules component and know where to redirect back. For that the module will have to provide some sort of UIAdapter which implements that logic.
- Current Rules UI elements will have to be converted to work without a config entity, but can work based upon a Rules component. So only surrounding UI parts will have the config entity to implement further specific UI like component variables UI or event attaching/removing. Those parts need not to be re-usable anyway.

Note: The embedded will contain links, which when pressed, will leading to the loss of other unsubmitted form values. We might be able to improve that in future by converting this links to some sort of ajax submissions.

Implementation plan

- Prepare a small testing module that demonstrates the usage: #2659026: Create testing module for embedding Rules UI
- Add support for defining rules_ui paths in yml files #2659932: Add support for defining rules_ui paths in yml files
- Convert Rules UI classes to work based on the Rules component: #2659028: Convert RulesUI to work based on RulesComponent
- Document API / usage
- Possibly provide an optional "Rules-Enhances Block UI" sub-module which swaps out core block conditions and replaces them with Rules condition.