Inline images in emails

[iudex-]

Hey,

I'm running vaultwarden in a VPN, so the linked images in the email don't load (as gmail tries to proxy them but can't find them)

I figured the images in the email header and footer might just use data URLs, but some googling shows that there are email clients, including gmail, that don't support this approach.

The correct approach seems to be the use of CIDs (Content-ID) and embedding the images as part of the multipart email.

I found this example using the lettre mailer library used by vaulwarden:
https://github.com/lettre/lettre/blob/master/src/message/mod.rs#L120-L145

Would you accept a PR adding this?
I have absolutely no clue about Rust, but if you pre-approve the idea I will make it happen 😅