Encrypt the Lambda function envirornment variables created using buildkite elastic stack

[RahulMahale]

The lambda function environment variables need to be encrypted as per the security standards.

While creating the Elastic CI stack we can force the encryption of the lambda environment variables with the default or the customer-managed key.

[moskyb]

hey there @RahulMahale! i wasn't aware that lambda had the capability for encrypting envars in transit, that's a nifty feature - docs here, for future travellers.

what security standards are you using that would require this? would you be interested in submitting a PR to implement this feature?