Add ability to use custom validations for a testbed file

[extsoft]

Since pyATS 5.0.0 we have the ability to validate a testbed file structure with pyats validate testbed my-testbed.yaml.

It will be great to have the ability to add custom rules for testbed validation. Use case: let's assume there is an additional keyword in a device's custom section. So, I want to provide validation options for this keyword to make sure it is specified correctly by someone who configures the testbed file. This allows me to validate data before running actual tests (and don't repeat some validations in the code again and again).