Warn and handle if AMS messages are > 1 MB

[tofu-rocketry]

Related to #74 as it could/should be implemented alongside that.

AMS has a limit of 1MB for messages as that is what it is optimised around. SSM should check the size of messages (and any associated wrapping) before sending them to check they won't hit this limit, and ideally handle a fail gracefully. #74 could then be used to split the message into manageable parts.

Should also consider what to do if any messages can't be sent - send what you can, or fail completely? Probably send what you can.

[tofu-rocketry]

Somewhat duplicates #155 though with the aim or pre-empting the problem.

[tofu-rocketry]

https://github.com/apel/ssm/blob/3.2.1-1/ssm/ssm2.py#L480

[kreczko]

Should also consider what to do if any messages can't be sent - send what you can, or fail completely? Probably send what you can.

Is there a way to split up the messages?
Currently, this completely blocks publishing if for some reason the message exceeds this limit.
Naively I would expect the accounting records to be easily partitionable