Add method reference to FuncOp

[mabbay]

Allow user to attach a MethodRef to a FuncOp.
Compiler tests are failing because of these changes, I will updated them gradually.
Attaching MethodRef to FuncOp will allow us to access the original j.l.r.Method from which we extracted a FuncOp -JDK-8376824

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[openjdk]

❗ This change is not yet ready to be integrated.
See the Progress checklist in the description for automated requirements.

[PaulSandoz]

I like that you reused the method reference, but I am unsure about using null as for the reference owner type, arguably MethodRef should throw NPE for any given null arguments. Perhaps we can use void?

[openjdk]

@mabbay this pull request can not be integrated into code-reflection due to one or more merge conflicts. To resolve these merge conflicts and update this pull request you can run the following commands in the local repository for your personal fork:

git checkout funcop-mref
git fetch https://git.openjdk.org/babylon.git code-reflection
git merge FETCH_HEAD
# resolve conflicts and follow the instructions given by git merge
git commit -m "Merge code-reflection"
git push

[mabbay]

/template append

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