Define Secret type

[mattt]

This PR defines a new Secret type in Cog's Python package, which model authors can use to signify that an input holds sensitive information (like a password or API token).

The Secret type inherits from Pydantic's SecretStr:

You can use the SecretStr and the SecretBytes data types for storing sensitive information that you do not want to be visible in logging or tracebacks. SecretStr and SecretBytes can be initialized idempotently or by using str or bytes literals respectively. The SecretStr and SecretBytes will be formatted as either '**********' or '' on conversion to json.

from cog import BasePredictor, Secret


class Predictor(BasePredictor):
    def predict(self, secret: Secret) -> None:
        # Prints '**********'
        print(secret)        

        # Use get_secret_value method to see the secret's content.
        print(secret.get_secret_value())

A predictor's Secret inputs are represented in OpenAPI with the following schema:

{
  "type": "string",
  "format": "password",
  "x-cog-secret": true,
}

Replicate will treat secret inputs differently throughout its system. For example, a prediction on Replicate will redact secret values in its JSON representation and log lines.

[technillogue]

what I really like about this is that it gives us to flexibility to potentially only redact and not store prediction secrets