feat: Provide users possible status codes that SendGrid can return

[maxkoshevoi]

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.

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[childish-sambino]

What happens to this property when the value is not defined in SendGridStatusCode? Guessing null and worried that clients would then not get the actual status code. Could this be added as an extra property of SendGridErrorResponse so it's backwards-compatible and supports the benefits of the enum?

[maxkoshevoi]

enums are value type, so cannot be null. If value of ErrorHttpStatusCode is not defined in SendGridStatusCode, it will still be stored in ErrorHttpStatusCode (Visual Studio will just show you int value instead of a name).

Admittedly, this might be not the best solution since value of ErrorHttpStatusCode might be outside of what's defined SendGridStatusCode, but no information will be lost.