[Thunderbird] use getRaw() for accessing message.asc

[martgil]

          What about using [messages.getRaw](https://webextension-api.thunderbird.net/en/latest/messages.html#getraw-messageid-options) for this case?

I tried to run getRaw with decrypt: false param and it returns full content of message.asc attachment.

Originally posted by @sosnovsky in #5828 (comment)

[martgil]

Hi @sosnovsky, I decided to make a separate issue for this one.