Introduce module instance context APIs which can set one or more contexts created
by the embedder for a wasm module instance:
```C
wasm_runtime_create_context_key
wasm_runtime_destroy_context_key
wasm_runtime_set_context
wasm_runtime_set_context_spread
wasm_runtime_get_context
```
And make libc-wasi use it and set wasi context as the first context bound to the wasm
module instance.
Also add samples.
Refer to https://github.com/bytecodealliance/wasm-micro-runtime/issues/2460.
