Running Extensions: add search and extension-host filter

## Feature Request
Add search and extension-host filtering to the **Running Extensions** editor (`Developer: Show Running Extensions`).

## Motivation
The Running Extensions list can get long. Today there is no way to:
- quickly find an extension by name/id
- filter to Local vs Remote vs a specific extension host (including affinity hosts)

## Proposal
- Sticky search bar at the top of the editor
- Dropdown filter: All Extension Hosts / Local / Remote / per-host entries

## Implementation
PR: https://github.com/microsoft/vscode/pull/328402