up: add templated if for operations

[XaF]

This will allow to select which operations to run depending on environment variables, prompts, etc.

This will be particularly useful to conditionally execute steps for local development or CI for instance.

if could be a layered operation, where it takes a "condition" parameter (template) or "condition_command" parameter (if we want to execute a bash script to decide for instance!) and "operations" which would default to a "and" operation for anything specified.

Alternatively, "if" and "if_command" could be added as parameters of any up operation.