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.

samples/inst-context-threads/wasm-apps/testapp.c

@@ -0,0 +1,65 @@
+/*
+ * Copyright (C) 2023 Midokura Japan KK.  All rights reserved.
+ * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+ */
+
+#include <assert.h>
+#include <pthread.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdint.h>
+
+void
+set_context(int32_t n) __attribute__((import_module("env")))
+__attribute__((import_name("set_context")));
+
+int32_t
+get_context() __attribute__((import_module("env")))
+__attribute__((import_name("get_context")));
+
+void *
+start(void *vp)
+{
+    int32_t v;
+
+    printf("thread started\n");
+
+    printf("confirming the initial state on thread\n");
+    v = get_context();
+    assert(v == -1);
+
+    printf("setting the context on thread\n");
+    set_context(1234);
+
+    printf("confirming the context on thread\n");
+    v = get_context();
+    assert(v == 1234);
+    return NULL;
+}
+
+int
+main()
+{
+    pthread_t t1;
+    int32_t v;
+    int ret;
+
+    printf("confirming the initial state on main\n");
+    v = get_context();
+    assert(v == -1);
+
+    printf("creating a thread\n");
+    ret = pthread_create(&t1, NULL, start, NULL);
+    assert(ret == 0);
+    void *val;
+    ret = pthread_join(t1, &val);
+    assert(ret == 0);
+    printf("joined the thread\n");
+
+    printf("confirming the context propagated from the thread on main\n");
+    v = get_context();
+    assert(v == 1234);
+
+    printf("success\n");
+    return 0;
+}