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

• [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
• [ ] I have added inline documentation to the code I modified

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