Improve wasm-c-api instantiation-time linking (#1902)

Add APIs to help prepare the imports for the wasm-c-api `wasm_instance_new`:
- wasm_importtype_is_linked
- wasm_runtime_is_import_func_linked
- wasm_runtime_is_import_global_linked
- wasm_extern_new_empty

For wasm-c-api, developer may use `wasm_module_imports` to get the import
types info, check whether an import func/global is linked with the above API,
and ignore the linking of an import func/global with `wasm_extern_new_empty`.

Sample `wasm-c-api-import` is added and document is updated.

core/iwasm/include/wasm_c_api.h

@@ -354,6 +354,7 @@ WASM_API_EXTERN own wasm_importtype_t* wasm_importtype_new(
 WASM_API_EXTERN const wasm_name_t* wasm_importtype_module(const wasm_importtype_t*);
 WASM_API_EXTERN const wasm_name_t* wasm_importtype_name(const wasm_importtype_t*);
 WASM_API_EXTERN const wasm_externtype_t* wasm_importtype_type(const wasm_importtype_t*);
+WASM_API_EXTERN bool wasm_importtype_is_linked(const wasm_importtype_t*);
 
 
 // Export Types
@@ -797,6 +798,9 @@ static inline void* wasm_val_ptr(const wasm_val_t* val) {
 
 #define KILOBYTE(n) ((n) * 1024)
 
+// Create placeholders filled in `wasm_externvec_t* imports` for `wasm_instance_new()`
+WASM_API_EXTERN wasm_extern_t *wasm_extern_new_empty(wasm_store_t *,  wasm_externkind_t);
+
 ///////////////////////////////////////////////////////////////////////////////
 
 #undef own

core/iwasm/include/wasm_export.h

@@ -1324,6 +1324,22 @@ wasm_runtime_get_custom_section(wasm_module_t const module_comm,
  */
 WASM_RUNTIME_API_EXTERN void
 wasm_runtime_get_version(uint32_t *major, uint32_t *minor, uint32_t *patch);
+
+/**
+ * Check whether an import func `(import <module_name> <func_name> (func ...))` is linked or not
+ * with runtime registered natvie functions
+ */
+WASM_RUNTIME_API_EXTERN bool
+wasm_runtime_is_import_func_linked(const char *module_name,
+                                   const char *func_name);
+
+/**
+ * Check whether an import global `(import <module_name> <global_name> (global ...))` is linked or not
+ * with runtime registered natvie globals
+ */
+WASM_RUNTIME_API_EXTERN bool
+wasm_runtime_is_import_global_linked(const char *module_name,
+                                     const char *global_name);
 /* clang-format on */
 
 #ifdef __cplusplus