Expose `key_value` in root module

[bobf]

I have an abstraction layer that wraps http.zig headers, currently I'm doing this:

const HttpzKeyValue = std.meta.fieldInfo(httpz.Request, std.meta.FieldEnum(httpz.Request).headers).type;

const MyHeaders = struct {
    httpz_headers: *HttpzKeyValue,
    // ...
};

If I'm missing something obvious that makes this PR unneeded please feel free to share.