fix: handle Uint8Array CipherTextBlobs

[HW13]

Summary

• Update awsKms to handle CiphertextBlob as a Uint8Array
  • Previously was handling it as a Buffer which is no longer the case (docs)
  • Update test stubs to return expected types
• Update type delcarations

Test plan

• Using yalc add this package to our auth project
• Implement awsKms and pass it to createEnvelopeEncryptor in the DB seeds
• Confirm seeds run without error