Symfony version(s) affected

5.4.2

Description

When trying to send a custom header towards AWS using the AsyncAws implementation custom headers aren't included.
This seems to be due to the usage of the Simple message format that doesn't seem to allow for custom headers:
https://docs.aws.amazon.com/ses/latest/APIReference-V2/API_SendEmail.html

Upon printing the Api request the headers are nowhere to be found either.

How to reproduce

Add a header to the mail via:
$message->getHeaders()->addTextHeader('x-test', 'test'));
and send it using the ses+api DSN

Possible Solution

There seems to be a Raw message format you can send that allows for custom headers so the package should probably use that.

Additional Context

No response