Provide sidecar filesystem access for VS Code extensions

[azatsarynnyy]

Is your task related to a problem? Please describe.

When a Che-Theia plug-in is running a separate sidecar and it requests a resource from a local file system with vscode.workspace.fs API, Che-Theia tries to look for a requested resource in its own container. But such a call should be forwarded into the plug-in's sidecar.

Describe the solution you'd like

We need to adapt vscode.workspace.fs Plug-in API to a remote plug-in model. A plug-in should be able to access the resources in its own sidecar file system.

Describe alternatives you've considered

Additional context

This issue is a part of #16818

The Devfile for testing includes the Python VS Code extension running in its own sidecar: https://gist.githubusercontent.com/azatsarynnyy/6bc5daaa984196569c961a8e8bb31459/raw/5002be117eeabfe8e2459b6f318f93655e11b009/devfile.yaml

The example of FS API call in Python extension: https://github.com/microsoft/vscode-python/blob/df3d6c7d00f2b4ec7bf2ae1032db4450882f44c8/src/client/common/platform/fileSystem.ts#L130

The example of how other APIs are adapted to a remote plug-in model: https://github.com/eclipse/che-theia/blob/5b151b0bc2aef303748f96b88b0e2afb89c26200/extensions/eclipse-che-theia-plugin-remote/src/node/plugin-remote-init.ts#L229

FS API in upstream Theia that should be adapted: https://github.com/eclipse-theia/theia/blob/4e1c1d429226280e6f0eff49658c733a38b50c4a/packages/plugin-ext/src/plugin/file-system-ext-impl.ts#L191

[azatsarynnyy]

It should work well after replacing the schema of a requested URI from local to file-sidecar-${machineName}.