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.
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