Refine optional vs required and default values

[martinbonnin]

Description

At the moment, it's not 100% clear how to handle required values.

By default, it outputs an error like this:

* What went wrong:
A problem was found with the configuration of task ':librarianPublishToGcs' (type 'PublishToGcsTask').
  - Type 'com.gradleup.librarian.gradle.internal.task.PublishToGcsTask' property 'credentials' doesn't have a configured value.

But I consider task properties to be mostly implementation details. It's a bit weird to have them leak in the error messages like this.

The alternative is to mark every input nullable and then handle the default in the task action but it's more work for the task action.