Proposal for new decorator to define HEAD operations

[catalinaperalta]

Clear and concise description of the problem

We recently worked with some HEAD operation definitions and thought that it would be useful to have a decorator that allows folks to decorate a given operation with an @headOp decorator, which would automatically signal that this service also supports HEAD operations for that endpoint and automatically considers all of the headers for that operation so the true definition lives in one place and it's easier to maintain the supported set of headers in one definition.

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[timotheeguerin]

just because a decorator creating a new declaration at the same level can be a bit messy a cleaner approach might be to have

@get op getBlob(): File;

op listProperties is Head<getBlob>;

with the new functions coming up this would be a great use case