Nest BaseInterface under sendgrid namespace

[adibsaad]

Fixes

The constant "BaseInterface" is used by my app, and when I upgraded Sendgrid to the latest version, it resulted in conflicts.
I figured the word "BaseInterface" is generic enough that it's used by many people and can lead to hard-to-find bugs, so I'm suggesting moving BaseInterface to SendGrid::BaseInterface

Checklist

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