Implement better detection of "multipart/encrypted" email with PGP encrypted message

[martgil]

Description: The FlowCrypt iOS didn't recognize "multipart/encrypted" email with PGP encrypted message. Emails at https://mail.google.com/mail/u/flowcrypt.compatibility@gmail.com/#inbox/FMfcgzGxTFXMFnDNbgknpqvXKtSmMgSK isn't recognized by the FlowCrypt app and shows not encrypted + no signed blank email thread.

Reference:

• https://github.com/FlowCrypt/flowcrypt-browser/pull/5727
• https://github.com/FlowCrypt/flowcrypt-android/blob/e58d040f8eade675ca8c6bd18cb608d46bcdb44b/FlowCrypt/src/main/java/com/flowcrypt/email/extensions/jakarta/mail/PartExt.kt#L82