Refactor externref related APIs of reference types feature (#971)

Currently when calling wasm_runtime_call_wasm() to invoke wasm function
with externref type argument from runtime embedder, developer needs to
use wasm_externref_obj2ref() to convert externref obj into an internal ref
index firstly, which is not convenient to developer.
To align with GC feature in which all the references passed to
wasm_runtime_call_wasm() can be object pointers directly, we change the
interface of wasm_runtime_call_wasm() to allow to pass object pointer
directly for the externref argument, and refactor the related codes, update
the related samples and the document.

core/iwasm/compilation/aot.h

@@ -96,6 +96,7 @@ typedef struct AOTMemInitData {
 typedef struct AOTImportTable {
     char *module_name;
     char *table_name;
+    uint32 elem_type;
     uint32 table_flags;
     uint32 table_init_size;
     uint32 table_max_size;