Add the ability to pass an external jsonschema file for validation.

_Originally posted by @stan-sz in https://github.com/Azure/bicep/issues/9228#issuecomment-1357338706_

In place of a type declared via an ARM template or Bicep, this feature would allow users to load an external JSON schema file.

The issue names no files, tests, or entry points. First clarify how an external JSON Schema should be supplied, which validation paths it covers, and how it interacts with ARM template or Bicep types; then locate the relevant validation entry points and define tests for successful loading, invalid schemas, and validation failures.

Written by the indexing model from the issue text.