[FEATURE] custom validation rules for buildDefinition & deploy & retrieve

[JoernBerkefeld]

based on

• [FEATURE] validation rules for retrieve, buildDefinition & deploy #1643

idea is that we offer an option to run a callback function (needs to be nicely encapsulated to not break mcdev) to offer full flexibility.

the hook should run

• at the end of the preDeployTasks to basically allow adding your own preDeployTasks. If the custom script fails, it should block deployment.
• in buildDefinition after all templating variables were applied. This way we can show warnings

the method parameters:

• metadata (or the code-json split version of it)
• target BU (cred/bu)

the method may be async - we shall put an await in front of it for good measure and a try catch around it to format the error message.

it shall either return the (potentially adapted) object it received as parameter OR throw a descriptive error.

in preDeployTasks, that error will NOT be caught, preventing deployment
in buildTemplate it will be caught and displayed as a warning, allowing the file to be saved in the deploy folder

the callback-javascript file path can be set via a new option in .mcdevrc, relative to the project root

[github-actions]

Closed by #1649.