Fixes

Add PlaybackGrant.

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[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
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[x] I have updated my branch with the main branch
[x] 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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twilio / twilio-go