[feat]: Consistent loading state for `Select` and `Combobox`

[gu-stav]

A clear and concise description of what the feature is

Add a isLoading property to Select and Combobox components, to render a spinner.

Why should this feature be included?

In the CMS we have to load a lot of data when rendering, which leads to loading spinners. The Select and Combobox components are not exceptions and often depend on data being loaded (e.g. for review workflows).

While it is possible to set a loading state for Select via customizeContent it is not for Combobox, and therefore when used together both components appear different during the loading phase.

Please provide an example for how this would work

see above