feat: add SendGridMessage deserialization

[jlawcordova]

Fixes #1104

Add a static method Deserialize to SendGrid.Helpers.Mail.SendGridMessage which recreates a SendgridMessage instance from a JSON object.

Checklist

• I acknowledge that all my contributions will be made under the project's license
• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the Contribution Guidelines and my PR follows them
• I have titled the PR appropriately
• I have updated my branch with the main branch
• I have added tests that prove my fix is effective or that my feature works
• I have added the necessary documentation about the functionality in the appropriate .md file
• I have added inline documentation to the code I modified

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[JenniferMah]

Thanks for your contribution @jlawcordova!