Allow creating secrets from environment variables and files in CLI

[danielgafni]

Currently the only way to set the secret contents is to pass them in CLI args in plain text.

This is:

• not ideal locally (the secret value is recorded in the shell history unless special precautions are taken)
• not acceptable in CI/CD pipelines which commonly display the command being run in the logs

I think the modal secret create command should have an interface to load secret values from environment or filesystem instead.

For example:

modal secret create my-secret MY_VALUE=env://MY_VALUE

modal secret create my-secret MY_VALUE=file:///path/to/value

modal secret create --from-file /path/to/secret.json my-secret

[mwaskom]

Thanks for the suggestion, interesting idea! Definitely agree that passing secrets in plaintext is not great. Currently we support what you want to do more through the programmatic secret API, e.g. modal.Secret.from_dotenv. Will think about how we can pull some of this support into the CLI too! Gonna track this as a feature request internally.