Improve `ComputationalResourcesWidget`

Currently, the `ComputationalResourcesWidget` is used to select the `code` with a `default_calc_job_plugin`. But not include `number of nodes`, `number of cpus` etc.

So either we add support for setting up the computer resource (nodes, cpus), or we change its name similar to `AiiDACodeWidget`.

Besides, there are some settings that are special to a code, for example, the parallelization levels (pools, images, nk, etc.) in the `pw.x` code. It would be better to expose some entry point that one can customize it.