[coepi-backend-aws]: Implement reCaptcha verification on the server side

[ramnanib2]

Currently, the apps send POST request to the back-end without any authentication, for privacy-preserving reasons. Performing server-side reCaptcha verification would prevent abuse, DOS attacks and fake report generation. We should also consider privacy-pass (https://www.hcaptcha.com/privacy-pass). This requires changes on the client side as well. So recommend doing this in v5