new feature: mount-like service

### Feature Description

I suggest a new service providing mountpoint-based binding to other services.

Each mount is a triple of `(path, service operator, prefix)`, declaring that access to files under `path` should be forwarded to `service operator`, with `path` in their paths replaced with `prefix`. File access should always match the mount with longest common ancestor.

For example,
- /, A, /
- /dir, B, /test
Accessing `/dir/a` should be the same as accessing `/test/a` with B, accessing `/b` should be the same as accessing `/b` with service A.

### Problem and Solution

This is because sometimes we may want to storage our data in different services, based on directories.

### Additional Context

_No response_

### Are you willing to contribute to the development of this feature?

- [ ] Yes, I am willing to contribute to the development of this feature.

The issue names no files, tests, or entry points. Start by surveying the repository's service abstractions and path-handling code, then identify where a mount service would fit. Done should include the specified longest-common-ancestor routing and path-prefix replacement, with tests covering the root and nested mount examples.

Written by the indexing model from the issue text.