Add support for sending raw MIME messages

[mrashed-dev]

Description

This PR adds support for sending raw MIME messages.

Usage

from nylas import APIClient
nylas = APIClient(
    CLIENT_ID,
    CLIENT_SECRET,
    ACCESS_TOKEN
)

raw_mime = """MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Subject: With Love, From Nylas
From: You <example@gmail.com>
To: My Nylas Friend <recipient@gmail.com>

This email was sent via raw MIME using the Nylas email API. Visit https://nylas.com for details.
"""

draft = nylas.drafts.create()

# Send the raw MIME
msg = draft.send_raw(raw_mime)

License

I confirm that this contribution is made under the terms of the MIT license and that I have the authority necessary to make this contribution on behalf of its copyright owner.

[codecov]

Codecov Report

Merging #243 (e7e96b0) into main (a1e3a73) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #243      +/-   ##
==========================================
+ Coverage   95.99%   96.01%   +0.02%     
==========================================
  Files          14       15       +1     
  Lines        1648     1657       +9     
==========================================
+ Hits         1582     1591       +9     
  Misses         66       66              

Impacted Files	Coverage Δ
nylas/client/client.py	96.13% <100.00%> (+0.02%)	⬆️
nylas/client/restful_models.py	97.12% <100.00%> (+0.01%)	⬆️
nylas/__init__.py	100.00% <0.00%> (ø)

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[dantonpimentel]

lgtm