Normalize signature mime types when filtering attachments in emails

What does this MR do?

Fixes signatures filtering in emails when a MUA mixes x- prefixed and non prefixed mime types. We normalize the content-types using the MIME library, already included in the project.

We have found that when replying to issues via email, some comments didn't get their S/MIME signature dropped by gitlab, when they should have after !25389 (merged) in 12.8.

We pinpointed this to the raw emails having a mix of declared mime types with and without x- prefix in the same message. In the sample below, notice how the declared protocol and attachment content-type do not match:

...
Content-Type: multipart/signed;
	protocol="application/x-pkcs7-signature";
	micalg=SHA1;
	boundary="----=_NextPart_000_03BB_01D609D9.B3D67D60"
...
Content-Type: application/pkcs7-signature;
	name="smime.p7s"
Content-Transfer-Encoding: base64
Content-Disposition: attachment;
	filename="smime.p7s"
...

RFC5751 Compatibility with Prior Practice of S/MIME covering S/MIME v3 links to RFC2311 for S/MIME v2, which states that x-pkcs7-signature should be covered for compatibility.

🛠 with ❤ at Siemens

/cc @bufferoverflow @rpaik

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Does this MR meet the acceptance criteria?

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• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
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