feat: Provide users possible status codes that SendGrid can return

[maxkoshevoi]

trafficstars

Fixes

SendGrid specifies exact status codes it could return with their respective meanings, but currently SDK doesn't provide this information to the user.

Added a SendGridStatusCode enum that would make writing code that reacts to specific status codes more readable.

Checklist

• [x] I acknowledge that all my contributions will be made under the project's license
• [x] 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
• [ ] 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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