Add encrypt_invoke_data arg to encryption middleware

[amh4r]

Add encrypt_invoke_data arg to encryption middleware. Its purpose is to make invoke data encryption opt-in, since it's technically a breaking change. This allows users to add invoke data encryption in a zero-downtime way:

1. Update the Python SDK.
2. Release app.
3. Set encrypt_invoke_data=True.
4. Release app.

We will remove the encrypt_invoke_data arg in a future release, effectively making encrypt_invoke_data=True the hardcoded behavior.