twilio: add alternate auth token for use during key rotation

[mastercactapus]

Description:
This PR adds the AlternateAuthToken field to the config allowing for key rotation without request failures.

To validate/perform a key rotation:

1. Verify calls and/or bi-directional SMS works
2. Generate secondary token in Twilio
3. Set the Auth Token to the new secondary, and the Alternate Auth Token to the current primary
4. Verify calls and/or bi-directional SMS works
5. Promote secondary to primary in Twilio
6. Verify calls and/or bi-directional SMS works
7. Clear Alternate Auth Token
8. Verify calls and/or bi-directional SMS works