allow "x-" options in readthedocs.yml configuration

[NiklasRosenstein]

This Pull Request will allow that options starting with x- can be added to the .readthedocs.yml configuration. This is useful for custom workflows where it makes sense to embed additional options in the Read the Docs configuration. The x- convention is also used by Docker-Compose, for example.

My personal use case for this is to embed the configuration for readthedocs-custom-steps in the configuration directly, instead of a having to use a separate .readthedocs-custom-steps.yml file.

[stsewd]

Thanks, we should update the docs about this in https://github.com/readthedocs/readthedocs.org/blob/master/docs/config-file/v2.rst#changes and https://github.com/readthedocs/readthedocs.org/blob/master/docs/config-file/v2.rst#supported-settings

Also, @readthedocs/core is this something we want to support?

[agjohnson]

I would vote no. I don't really want 2 separate tools using the same configuration file. I'd sooner support native custom build steps, I'm not sure it needs to be a separate extension.

[humitos]

I'm closing this PR. We are moving in a different direction to support custom commands. There are some discussions about this at #8215 #8190