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/src/my_context.h

@@ -0,0 +1,11 @@
+/*
+ * Copyright (C) 2023 Midokura Japan KK.  All rights reserved.
+ * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+ */
+
+struct my_context {
+    int x;
+};
+
+extern void *my_context_key;
+extern struct my_context my_context;