proposal: consider inlining "github.com/sendgrid/rest"

[nikolaydubina]

this package has no go module.
so we don't easily known what it imports.

reading code I see it imports github.com/sendgrid/rest.
but it also has no go module.
also not clear what it imports.
the meat of that package is only couple lines of code, and lots of documentation.

what's more important, hashes of dependencies and versions not pinned.

this obscurity makes us not to use official sendgrid go client.

consider removing all unnecessary dependencies to improve outlook of this client.