Alias Parameter Transformations

[amanda11]

Following on from #1704 then although immutable alias parameters have simplified and removed the need in many cases for intermediate actions, I still have use-cases where it would be beneficial if the alias parameters could be transformed before they are sent to the action.

At the moment for example if the action takes a dictionary as a parameter, but I am only ever going to send it with the value for one key then I either have to:

• make the user send in the full dictionary in their alias command

• write a intermediary action to format the value before sending onto the main action.

This seemed to have been discussed under #1704 alongside default parameter values, but as far as I'm aware only default parameter values have been added (with the example of stripping off http).

For example, I currently have simplified aliases for the aws_boto3.boto3_action, and currently the user has to send in a full dictionary for params on their ! command on slack. When really I just need them to specify the value for a single key in that dictionary.