idea: Provide OpenDAL as host functions for wasm runtimes

[Xuanwo]

As demonstrated in @Zheaoli's opendal-wasmedge-demo, we can offer opendal as host functions for all of wasmtime, wasmedge and wasmer.

We can have the following design:

• opendal will provide an opendal-wasm-sdk that exposes our wasm API. Users can use the sdk across different supported runtime.
• opendal will provide host function modules for different wasm runtimes in opendal-wasm-module.

With those crates support, users can use opendal in this way:

• Import opendal-wasm-sdk or calling opendal wasm api in their wasm code.
• Using opendal-wasm-module to register host functions.
• Load wasm code.

---

There are many ways to integrate with WASM, and OpenDAL plans to support for the following:

• Compile to wasm32-unknown-unknown directly so users can use opendal in browser, tracked at Tracking issues of OpenDAL wasm32-unknown-unknown target support #3803
• Compile to wasm32-wasi directly so users can use opendal in wasm runtime like wasmtime and wasmedge. (need to use crates like tokio-wasi, not started)
• Split into host functions and client sdk, users will need to load functions when they start the runtime (this issue).
• Integrate into runtimes directly that replace their fs implementation. (Is this possible?)

[Zheaoli]

Maybe I can propose more design details this week.

BTW, about the opendal-wasm-sdk, we can take a look at proxy-wasm-rust-sdk, FYI https://github.com/proxy-wasm/proxy-wasm-rust-sdk

[Xuanwo]

@Zheaoli also mentioned a new way: https://wasmedge.org/docs/contribute/source/plugin/process

Provide a wasmedge plugin so users can:

wasmedge --dir .:. --reactor --process_plugin libopendal_wasmedge_plugin.so your_wasm_file.wasm