public CorsServiceBuilder allowRequestHeaders(Iterable<? extends CharSequence> headers) {
        requireNonNull(headers, "headers");
        firstPolicyBuilder.allowRequestHeaders(headers);
        return this;
    }
Specifies the headers that should be returned in the CORS {@code "Access-Control-Allow-Headers"}
response header.

<p>If a client specifies headers on the request, for example by calling:
<pre>{@code
xhr.setRequestHeader('My-Custom-Header', 'SomeValue');
}</pre>
the server will receive the above header name in the {@code "Access-Control-Request-Headers"} of the
preflight request. The server will then decide if it allows this header to be sent for the
real request (remember that a preflight is not the real request but a request asking the server
if it allows a request).

@param headers the headers to be added to the preflight
               {@code "Access-Control-Allow-Headers"} response header.
@return {@link CorsServiceBuilder} to support method chaining.
core/src/main/java/com/linecorp/armeria/server/cors/CorsServiceBuilder.java