Add support for Flint schemas and validations

[acalejos]

This PR does the following:

• Adds Flint as an optional dependency
• If Flint is present, creates the Instructor.Instruction module as a Flint extension
  • This will register the :doc option that can be used within embedded_schema fields
  • It will also create a default callback implementation for validate_changeset that validates the changeset according to the Flint extensions pipeline for the module
• Switches the order of the cond conditions when checking for validate_changeset callbacks such that if botharities of callback are available, then the arity-2 callback (that takes the context) takes priority
• Updates the json schema generation logic to use the :doc field options and moduledoc only if available. It maintains supports for the existing behavior