We add ref.func and ref.is_null to the fuzzed module.
ref.is_null returns i32, so it is added to i32 generator.
ref.func is added to GenerateOptRef.
GetRefType function is added to generate reftypes.

Bug: v8:11954
Change-Id: Ia1add950bed573a02b6bec1cba401273d401919e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3106925
Commit-Queue: Rakhim Khismet <khismet@google.com>
Reviewed-by: Manos Koukoutos <manoskouk@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76431}