feat: add HEAD handler

[teemingc]

closes #9468

Allows exporting the HEAD handler. Not sure if language-tools needs to be updated to remove the error message for this as well.

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[benmccann]

@s3812497 it looks like this PR will need a rebase

[teemingc]

Thanks for the reminder @benmccann . I've just learnt what a rebase is. I did a merge before that (hence the force push trying to undo that). Will use rebase from now on.

[benmccann]

Oh, I just use the term rebase out of habit. But really I think folks prefer if you do a merge because if they have the code checked out it's easier to pull a merge

[dummdidumm]

@s3812497 this has a merge conflict - can you look at it today? Else I'll look into it later.

[teemingc]

@s3812497 this has a merge conflict - can you look at it today? Else I'll look into it later.

Sure thing! I'll have a look at it now.

[benmccann]

I wonder if we'd want some sort of fallback implementation which executes GET and returns just the headers or if that defeats the purpose? I've never used HEAD before

[dummdidumm]

That fallback already exists today, and will still apply if you don't define the HEAD yourself.