feat: msg and email metadata (#3444)

Update partition_eml and partition_msg to capture cc, bcc, and message
id fields.

Docs PR: https://github.com/Unstructured-IO/docs/pull/135/files

Testing
```
from unstructured.partition.email import partition_email
from test_unstructured.unit_utils import example_doc_path

elements = partition_email(filename=example_doc_path("eml/fake-email-header.eml"), include_headers=True)
print(elements)
elements[0].metadata.to_dict()
```

Note to reviewers:
Tests in `test_unstructured/partition/test_email.py` were refactored and
rearranged to group similar tests together, so it will be easiest to
review those changes commit by commit.

---------

Co-authored-by: ryannikolaidis <[email protected]>
Co-authored-by: Coniferish <[email protected]>

Author: Coniferish

CHANGELOG.md

@@ -6,8 +6,8 @@
 
 ### Features
 
+* **Update partition_eml and partition_msg to capture cc, bcc, and message_id fields** Cc, bcc, and message_id information is captured in element metadata for both msg and email partitioning and `Recipient` elements are generated for cc and bcc when `include_headers=True` for email partitioning.
 * **Mark ingest as deprecated** Begin sunset of ingest code in this repo as it's been moved to a dedicated repo.
-
 * **Add `pdf_hi_res_max_pages` argument for partitioning, which allows rejecting PDF files that exceed this page number limit, when the `high_res` strategy is chosen.** By default, it will allow parsing PDF files with an unlimited number of pages.
 
 ### Fixes

example-docs/eml/fake-email-header.eml

@@ -1,13 +1,15 @@
 Received: from ABCDEFG-000.ABC.guide (00.0.0.00) by ABCDEFG-000.ABC.guide
  ([ba23::58b5:2236:45g2:88h2]) with Unstructured TTTT Server (version=ABC0_0,
  cipher=ABC_ABCDE_ABC_NOPE_ABC_000_ABC_ABC000) id 00.0.000.0 via Techbox
- Transport; Wed, 20 Feb 2023 10:03:18 +1200
+ Transport; Wed, 20 Feb 2023 10:03:18 +1200
 MIME-Version: 1.0
 Date: Fri, 16 Dec 2022 17:04:16 -0500
+Bcc: Hello <[email protected]>
 Message-ID: <CADc-_xaLB2FeVQ7mNsoX+NJb_7hAJhBKa_zet-rtgPGenj0uVw@mail.gmail.com>
 Subject: Test Email
 From: Matthew Robinson <[email protected]>
 To: Matthew Robinson <[email protected]>
+Cc: Fake Email <[email protected]>, [email protected]
 Content-Type: multipart/alternative; boundary="00000000000095c9b205eff92630"
 
 --00000000000095c9b205eff92630