feat: Add support for multiple Reply-Tos

[davidangb]

unit tests fail on LicenseTest.testLicenseShouldHaveCorrectYear(). I do not believe updating the license should be part of this PR. See #773

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Closes #696

Fixes

Add support to add multiple reply to email addresses.

Includes both a setReplyToList() setter and a addReplyTo() helper; the latter inspired by the C# client.

For context, see:

• support to add multiple reply to email addresses in the C# library: https://github.com/sendgrid/sendgrid-csharp/pull/1169
• support to add multiple reply to email addresses in the Node library: https://github.com/subinoy7/sendgrid-nodejs/pull/1/files
• a similar PR (abaondoned?) on sendgrid-java, but which does not have tests or examples: https://github.com/sendgrid/sendgrid-java/pull/737
• documentation for reply_to_list in the API request body: https://www.twilio.com/docs/sendgrid/api-reference/mail-send/mail-send#request-body

Checklist

• [x] 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)
• [x] I have read the Contribution Guidelines and my PR follows them
• [x] I have titled the PR appropriately
• [x] I have updated my branch with the main branch
• [x] 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
• [x] I have added inline documentation to the code I modified

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