* feat: influence parameter defaults through cli flag/env
Add a --parameter-default flag / CODER_RICH_PARAMETER_DEFAULT
environment variable which overrides default values suggested for
parameters.
This allows scripts or middleware wrapping the CLI to substitute
defaults for parameter values beyond those defined at the template
level. For example, Git repository/branch parameters can be given
defaults based on the current checkout, or default parameter values can
be parsed out of files inside the repo.
* Rename defaults arg to defaultOverrides
