Make CTFd category and difficulty mapping cutomizable through variables

[The0mikkel]

Update variables to support customizing the CTFd manager category and difficulty mapping.

Tasks

• Add Terraform variable to contain category and difficulty mapping
• Add variable to CTFp CLI
• Document variable

Implementation suggestion

For the variable to customize the category and difficulty mapping, the variable can utilize the map type.
This type allows to specify a near identical mapping, as what is used in the application.

It is possible to add validation to the variable, to validate the format.