Improved loading and parsing PGPMime Encrypted messages

[DenBond7]

This PR improved loading and parsing PGPMime Encrypted messages + Refactored code

close #3017

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Tests (delete all except exactly one):

• Tests added or updated

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To be filled by reviewers

I have reviewed that this PR... (tick whichever items you personally focused on during this review):

• addresses the issue it closes (if any)
• code is readable and understandable
• is accompanied with tests, or tests are not needed
• is free of vulnerabilities

[DenBond7]

I think test case for attachment with name msg.asc together with the noname file is not added?

The main changes here are

FlowCrypt/src/test/java/com/flowcrypt/email/security/pgp/ProcessMimeMessageTest.kt covers parsing cases like attachment with name msg.asc together with the noname file(PGPMime Encrypted)

But I think you are right. It would be nice to have a UI test for PGPMime Encrypted for Gmail API(conversation view). I will add it.