[VSA] Show record count for the stage (FE)

This is the frontend implementation issue. The backend implementation issue is #324687 (closed).

Problem to solve

Currently VSA displays a maximum of 20 records when a stage is selected. It does not display the total count anywhere. So if there are more than 20 records the total count is unknown. Even if there are fewer than 20 records in the stage, a user must count them manually to determine the total.

Proposal

VSA should display the record count for the stage. Probably using the 1001 approach.

Let's add the count information to the following places:

table header	stage popover
Add a badge with count in table header next to the Issue header	Add the count information to the stage popover that is displayed when users hover over a stage

Note: We should fetch count data asynchronously and start fetching data for all stages once the page has loaded. If necessary we should use loading indicators during load time.