feat: add headers support to sendgrid mail

[davidcheung]

ability to specify headers, so can thread messages using in-reply-to and any other headers sendgrid support for SMTP headers

[bmonkman]

For some reason it's appending _inner to a couple models during code generation which is why vet is failing:

[main] INFO  o.o.codegen.InlineModelResolver - Inline schema created as SendBulkMailResponse_successful_inner. To have complete control of the model name, set the `title` field or use the inlineSchemaNameMapping option (--inline-schema-name-mapping in CLI).
[main] INFO  o.o.codegen.InlineModelResolver - Inline schema created as SendBulkMailResponse_failed_inner. To have complete control of the model name, set the `title` field or use the inlineSchemaNameMapping option (--inline-schema-name-mapping in CLI).

[davidcheung]

Yeah looks like newer generator generates different code from the API specs, the build-and-push will pull the latest so its different from our dockerfile which uses FROM openapitools/openapi-generator-cli:v5.0.0-beta3 AS generate