Constraint declaration

[exavolt]

Check https://github.com/rez-go/constraints .

The user-flow would be something like this:

• Declare the constraint for a field as a variable in the code
• Provide this constraint through the skeleton
• The docgen will use the constraint provided through the skeleton to provide an accurate documentation for a field
• The function that uses the config, uses the same constraint to check the validity of the value of the field

We can make the loader to use the constraints provided through the skeleton when loading to automatically check the loaded values, but we prefer that validity-checking is performed by user functions to ensure that they always check for bad inputs.