feat: allow random auth token variable as an option

[travertischio]

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what

• Add a two variables create_random_auth_token and random_auth_token_length that would be used to generate a random auth token that is stored in terraform state and used as the auth_token for redis

why

Currently users of this module need to either handle the auth_token through something more secret outside of this module or commit the auth_token directly to their terraform variables. This random password path is a highly used pattern for modules like this one