Add general `cmd` and `args` for tools object

[colonesej]

Is your feature request related to a problem? Please describe.

There are a few requests to customise the command and the arguments for some tools and data types, like mars and copy.

Describe the solution you'd like

I think we can generalise it and provide a common interface for any data and tool config entry

data:
  type: mars
  cmd: /path/to/non/standard/mars_client

tools:
  packages:
    mypackage:
      type: git
      args: ["--origin github"] 

Describe alternatives you've considered

No response

Additional context

No response

Organisation

No response