Add default serialization for cookies in Custom Authorizer v2 Request

[themmes]

Context

When testing my CustomAuthorizer I got the missing 'cookies' field parsing error. It seems to be a required attribute without default setting, however it is not (always) provided by API Gateway.

Changes

• Added the serde default annotation to let Serde assume an empty cookie Vec in case the cookie attribute is not part of the request
• Added an example json without the cookie attribute
• Added a test for an example request json without the cookie attribute
• Fixed the test above that implied (the test function name) to be testing the same struct, but was actually using a different struct within the test

I hope this PR adds value, let me know if it raises any questions!

[calavera]

Thanks a lot for this fix! 🙌

[themmes]

You're very welcome, the project has been very useful so far!