Allow passing extra headers via meta

[christiaanwesterbeek]

Postgrest supports calling functions with a single JSON parameter by sending the header Prefer: params=single-object with your request.

This PR allows you to add any kind of header to your request using RA4's meta parameter. I chose this solution instead of something like a meta.singleJsonParameter boolean, because allowing to add any header makes the react-admin app more flexible, because many other Postgrest features are also activated by sending an additional header.

Example usage:

const [create, { isLoading, error }] = useCreate(
    'rpc/my-function',
    {
        data: { ... },
        meta: { headers: { Prefer: 'params=single-object' } },
    }
);

I'm open for discussions or making adjustments to this PR...

Btw I noticed code formatting is not enforced by something like prettier in this repo, so I tried to format my changes the best that I could to adhere to the current formatting.