Implement module instance context APIs (#2436)

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.

core/iwasm/interpreter/wasm_runtime.h

@@ -212,6 +212,7 @@ typedef struct CApiFuncImport {
 
 /* The common part of WASMModuleInstanceExtra and AOTModuleInstanceExtra */
 typedef struct WASMModuleInstanceExtraCommon {
+    void *contexts[WASM_MAX_INSTANCE_CONTEXTS];
     CApiFuncImport *c_api_func_imports;
     /* pointer to the exec env currently used */
     WASMExecEnv *cur_exec_env;
@@ -299,12 +300,8 @@ struct WASMModuleInstance {
        it denotes `AOTModule *` */
     DefPointer(WASMModule *, module);
 
-#if WASM_ENABLE_LIBC_WASI
-    /* WASI context */
-    DefPointer(WASIContext *, wasi_ctx);
-#else
-    DefPointer(void *, wasi_ctx);
-#endif
+    DefPointer(void *, used_to_be_wasi_ctx); /* unused */
+
     DefPointer(WASMExecEnv *, exec_env_singleton);
     /* Array of function pointers to import functions,
        not available in AOTModuleInstance */