Allow full parameter and input definition via valohai-utils prepare()

[JuhaKiili]

Today, you can only define:

parameter name, type, and default value. input name and default value.

Instead of passing just the dict of name and default value key-pairs, we should also allow for passing dict of dicts, where you can define all the other things like optionality, pass-as, keep-directories, etc.

Example:

default_parameters = {
  "learning_rate": {
    "default": 0.001, 
    "type": "float",
    "pass-as": "pass-as: --lr={v}",
    "description": "Initial learning rate"
  }
}