Add a way to show a progress indicator while fetching items for combobox

[jattasNI]

🙋 Feature Request

😯 Problem to Solve

This is the equivalent of #1311 but for the combobox. Some clients may need to do a network request to request combobox items based on the current text. While waiting for that request to complete they would like to show a spinner that indicates something is happening.

At this point we're not aware of specific clients that need this. We're planning to migrate many existing Nimble comboboxes and Material autocompletes to use the new select features and this might negate the need for it or might reveal that some use cases require it.

💁 Proposed Solution

The API should likely follow what we did for select, but there are complications around component differences like the fact that combobox has different autocomplete modes while the select has different search/filter modes. One idea is to remove the autocomplete modes from the combobox and align the API more closely with select filtering.

📋 Tasks