[Feat]: Support pending state for Combobox

[Rocss]

Code of conduct

• I agree to follow this project's code of conduct.

Impacted component(s)

sp-combobox

Description of the requested feature

Similar to sp-picker, sp-combobox should support a pending property which puts the component in a loading state.
The style for this is available in Spectrum CSS.

Mockups or screenshots

Implementation notes or ideas

When in a pending state, a combobox:

• receives focus, but does not open the option list on click, type, or arrow-down
• displays a loading spinner using sp-progress-circle, which has an aria-label provided via pendingLabel property
• allows typing, but does not display the list of autocompleted options when autocomplete=list