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