dynamic Resources: use VFS (virtual filesystem) instead of virtual doc

see https://github.com/aws/aws-toolkit-vscode/pull/2349#discussion_r763518772

Using `VirualFileSystem` with a custom scheme could avoid some quirks with virtual documents.

Why:

- flexible, allows use of vscode events `onFileSystem:s3`, `onFileSystem:s3-readonly`
- ..

Start with the linked pull request discussion and trace the dynamic Resources implementation that currently uses virtual documents. Compare that flow with VS Code's VirtualFileSystem and custom-scheme events such as onFileSystem:s3 and onFileSystem:s3-readonly. Done means the resource flow uses the VFS approach and avoids the listed virtual-document quirks.

Written by the indexing model from the issue text.