Add User Definable Variable Substitution

[Gavinok]

This PR resolves #649

I have documented and added a user definable way to add new variable substitutions. A simple API for defining regex based variable matching. Here is a simple example usage.

def sub_days_plus_one(days: str) -> int:
    """Strings like '$sub_days_plus_one_4' will be replaced with the
    final number incremented by one. In this case 5.
    $sub_days_plus_one_4 -> 5
    $sub_days_plus_one_10 -> 11"""
    return int(days) + 1


variable_substitution_map.add_variable_substitution(
    r"\$sub_days_plus_one_(\d+)", sub_days_plus_one
)
variable_substitution_map["$sub_days_plus_one_1"] # -> 2

[esune]

I do like how this is turning out. We need to add the override file bits to the Helm chart as well, maybe with a basic empty file definition that could be customized?

[loneil]

Think this works for me, good idea to have sample functions in there that would be in effect for a deployment. Yeah same question about what helm setup would be

[Gavinok]

I am going with Emilano's idea for now. I will setup the project with an empty python file that will be loaded allowing the users to extend it as they please

[esune]

I think this works, thanks @Gavinok

@loneil if you want to give it a review in case I missed anything I think we could merge after that.

[loneil]

I think this will work deployment-wise. Probably just put it through it's paces on a dev deploy and make sure