S3 bucket dropdown should have Search By Prefix support

## Problem
Currently, the S3 section of the AWS toolkit is not realistically usable for any bucket that has more than ~100 objects.

Workflows such as "find a text object by name out of the couple million objects in the bucket, edit it in vs code, and sync it back" are not viable because the object simply cannot be located - the UI lets the user paginate and fetch the next page of records, so getting to an object that is on the 1000th page would take unrealistic time.

## Expected behavior
The bucket dropdown should have a search field that filters down the result set by prefix, just like the S3 AWS console does.

Start in the AWS Toolkit's S3 section and locate the bucket dropdown that currently paginates objects. Review how bucket contents are fetched and compare the AWS console's prefix-search behavior. Done means a user can enter a prefix to narrow a large bucket's objects and select the matching object without paging through every result.

Written by the indexing model from the issue text.