We use the X-Leap-Signature and X-Leap-Encryption to signal the
signature and encryption status of emails. An attacker could add this
headers and trick bitmask to believe that the email was signed and/or
encrypted. Now we remove this headers from the original email if they
are present before adding ours.

- Resolves: #7429

It changes the way that incoming service checks if a additional
verification is needed to validate signature. The way before was
checking by the type of signature object and calling the verify
signature method if the type is different from OpenPGPKey. However it
could be more readable if we check the type of decrypted message. If
it's a multipart/signed message and not a plain/text we need to verify
the signature because keymanager couldn't do it during the decryption
process.

This change adds test to validate signature of encrypted email created
by apple mail. It's important to note that apple mail has a specific way
to encrypt signed messages. First it sign the email and then encrypt the
previous signed message. It was also added a message file with the
expected data.

- Related: #8031

The functions decryption_error_not_called and add_decrypted_header_called
were not being called on testDecryptEmail. So the asserts was not being
called as well.

This change adds the above functions as callbacks to be called after the
fetch method.

Fix verify signature on encrypted email from Apple Mail, adding a step
to verify signature after decrypt the email because the keymananger
could not verify signature when decrypting it

for consistency with the previous Account change.

- Resolves: #7977