Fixes

Motivation
This PR addresses an inconsistency in the TypeScript type definition for the Attachment model that prevents proper use of TypeScript with the library.

Issue
The Attachment model's TypeScript type definition does not match the expected data structure for the SendGrid API. Specifically, the contentId field should be content_id to align with the API requirements. This discrepancy does not impact JavaScript users but hinders TypeScript usage by causing type errors.

Solution
I store attachment details in the appropriate class with the toJSON method to ensure the content_id field is correctly included in the API request payload.

Also, I added tests to confirm the content_id field is correctly handled and included in the API request payload.

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