feat(services): Add `wasi-fs` backend using `wasi:filesystem` interface

[JMLX42]

Summary

Add a new OpenDAL backend that leverages the wasi:filesystem interface for file operations. This enables OpenDAL to access the host filesystem when running as a WebAssembly component, with the host runtime (e.g., wasmtime, wasmer) providing the actual filesystem implementation.

Motivation

Currently, the services-fs backend cannot compile to wasm32-wasip2 because it depends on tokio's filesystem features, which are not supported on WASM targets.

However, WASM components can access the host filesystem through the standardized wasi:filesystem interface. A wasi-fs backend would:

1. Enable filesystem access in WASM components — Components could read/write files preopened by the host runtime
2. Complement existing backends — memory and s3 work on WASM, but there's no local filesystem option
3. Leverage existing WASI ecosystem — Runtimes like wasmtime already implement wasi:filesystem

Proposed Implementation

A new services-wasi-fs feature/backend that:

• Imports wasi:filesystem/types and wasi:filesystem/preopens interfaces
• Uses wasi:io/streams for read/write operations
• Works with preopened directories provided by the host
• Compiles to wasm32-wasip2 target only (or conditional compilation)

Use Case

We're building WASM components that expose storage capabilities. We want to:

1. Create an OpenDAL-based storage component
2. Export wasi:filesystem interface for downstream consumers
3. Support multiple backends (memory, S3, and host filesystem)

Without a wasi-fs backend, components cannot access files that the host runtime preopens for them.

Research Notes

Backend	WASM-compatible	Notes
services-memory	✅	Works out of the box
services-s3	✅	Works with reqwest/reqsign
services-fs	❌	Requires tokio fs (not WASM-compatible)
services-wasi-fs	🆕	Proposed — uses wasi:filesystem imports

Related

• WASI filesystem spec: https://github.com/WebAssembly/wasi-filesystem
• Preview 2 (Component Model): https://github.com/WebAssembly/WASI/blob/main/preview2/README.md