Make nova-2 and aura-asteria-en the Default Models

[dvonthenen]

Proposed changes

If a model is not specified, use nova-2 and aura-asteria-en as the default models for STT and TTS respectively. This is needed in order to make vocode integration a little easier, plus there isn't any inherent problem with defaulting to nova-2.

Also, updated all examples to use nova-2.

Tested to make sure the default is set correctly and examples still work.

Types of changes

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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update or tests (if none of the other choices apply)

Checklist

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• I have read the CONTRIBUTING doc
• I have lint'ed all of my code using repo standards
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

NA