Alb multivalueheaders

[adrienblot]

When using AWS ALB with multi-value headers enabled, the Lambda event payload does not include a headers property, and it ignores the headers property in the response payload.

This PR introduces a new configuration option: albMultiValueHeaders.

When enabled, all headers from the request’s multiValueHeaders property are passed through to the Fastify request object.

Likewise, all headers set in the Fastify response are added to the return payload’s multiValueHeaders as arrays.

Checklist

• run npm run test && npm run benchmark --if-present
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[adrai]

thank you... included in v6.1.0