get_body() is not robust to LF line endings

[link2xt]

Current code of get_body expects that body starts after <CR><LF><CR><LF>:

dkim/src/hash.rs

Line 23 in b50a082

Ok(bytes::get_all_after(email.raw_bytes, b"\r\n\r\n").to_vec())

This code does not work for mails extracted from Dovecot Maildir which have LF line endings.

mailparse already splits headers and body in a different way inside parse_headers function:
https://github.com/staktrace/mailparse/blob/c51e92416c193072c739035fdcd5501c15f690e4/src/lib.rs#L939C1-L952C7

I managed to verify signatures of mails from Maildir, but had to do .replace("\n", "\r\n").

It would be nice to reuse mailparse splitting instead of splitting in a non-robust way. mailparse currently does not expose body_bytes (and header_bytes) fields, but proposing this to https://github.com/staktrace/mailparse/ is easy as it is well-maintained.