Add S3 object name search / keyword filter in AWS Toolkit for VS Code

The AWS Toolkit for VS Code currently allows browsing S3 buckets and prefixes,
but it is missing a critical feature: the ability to **search/filter S3 object names**.

For buckets with large numbers of objects (logs, data lake files, archives, etc.),
expanding the tree manually is extremely inefficient and not feasible.

Requested features:

1. A search box in the S3 Explorer panel
2. Keyword-based search (contains / startsWith)
3. Clickable search results that open or reveal the object in its prefix
4. Basic paging or lazy loading for large buckets

This feature is highly requested by many developers and would greatly improve
the usability of the Toolkit for S3-related workflows.

Thanks for your great work!

The issue names no files, tests, or entry points. Start by locating the S3 Explorer panel and its existing bucket and prefix browsing implementation. Done means the panel supports object-name keyword filtering, clickable results, and paging or lazy loading for large buckets.

Written by the indexing model from the issue text.