Fix union in header query issue

[qiaozha]

fixes #2185

[qiaozha]

Hold this PR as there's some issues in the modular layer.

[qiaozha]

Moving the isKey readOnly logic fix and support named union in Modular layer into this PR.

[MaryGao]

LGTM

[qiaozha]

In this case, we actually have response headers returned, but we just return void in the modular layer, I wonder if we should rethink about this a little ? @xirzec @joheredi

[qiaozha]

here's the typespec

      op get(
        @header("test-header") testHeader: "A" | "B",
        @body body: string,
      ): { @header("test-header") testHeader: "A" | "B" };

[qiaozha]

create an issue #2202 to track this.

[qiaozha]

We can fix remaining issue in other PRs.