regenerate sesv2

[mklinik]

Amazon recently, end of May or so, added new features to the SESv2 sendEmail endpoint. You can now specify email headers even in the Simple format https://docs.aws.amazon.com/ses/latest/APIReference-V2/API_Message.html. The data model already has the API spec with the Headers field: https://github.com/boto/botocore/blob/develop/botocore/data/sesv2/2019-09-27/service-2.json#L5096

Please regenerate the sesv2 api to make this available in amazonka.

[endgame]

Blocked by #888 — we can't advance the botocore pin until the generator is fixed because it will break some delicate, order-dependent code inside and generate service bindings that fail to compile.