Add Manage Domains API and Nylas service account request signing

[pengfeiye]

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.

Summary

• Adds Manage Domains support for /v3/admin/domains via Client.domains, with models in nylas.models.domains and methods: list, create, find, update, delete, get_info, and verify.
• Adds ServiceAccountSigner in nylas.handler.service_account and an optional signer argument on each Domains method to set X-Nylas-Kid, X-Nylas-Nonce, X-Nylas-Timestamp, and X-Nylas-Signature, using canonical JSON for request bodies where required.
• Adds a cryptography dependency and extends HttpClient._execute with optional serialized_json_body so signed payloads match the bytes sent on the wire.
• Adds tests for domains, service-account signing, and client.domains; updates CHANGELOG.md (Unreleased).

[codecov]

Codecov Report

❌ Patch coverage is 81.73077% with 38 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.51%. Comparing base (00e3f81) to head (1a812b7).
⚠️ Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
nylas/resources/domains.py	59.52%	34 Missing ⚠️
nylas/handler/api_resources.py	77.77%	2 Missing ⚠️
nylas/handler/http_client.py	80.00%	1 Missing ⚠️
nylas/handler/service_account.py	98.18%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #466      +/-   ##
==========================================
- Coverage   98.90%   97.51%   -1.40%     
==========================================
  Files          52       57       +5     
  Lines        2293     2579     +286     
==========================================
+ Hits         2268     2515     +247     
- Misses         25       64      +39     

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