* feat: Make validate command detect when an unknown backend type is in use.
* feat: Make validate command detect when the backend configuration doesn't match the schema.
* fix: Stop suppressing the Required:true parts of the backend schema when validating backend blocks
* test: Add test showing validation fails when a required attribute is missing from a backend's config
* test: Show how the validate command doesn't detect when an unknown backend type is in the config.
* test: Minor fixes to error messages when test fails
* test: Add some TODOs in code comments
These tests are here to define existing behaviour, not define desired behaviour. I wanted to make that clear!
* test: Fix typo in test name
* test: Show how the validate command validates resource blocks using their schema.
* test: Show how the validate command doesn't use backends' schemas when validating backend blocks.
