Add support for external recipes

[radoslaw-panuszewski]

Context

This PR adds support for registering external JARs to extend allwrite with additional recipes from any URL.

New commands

Add an external recipe JAR:

allwrite external add custom-recipes https://repo.com/custom-recipes-1.0.0.jar

Update an external recipe JAR with a new URL:

allwrite external update custom-recipes https://repo.com/custom-recipes-2.0.0.jar

Re-fetch an external recipe JAR from its stored URL:

allwrite external update custom-recipes

List all registered external recipe JARs:

allwrite external ls

Remove an external recipe JAR:

allwrite external rm custom-recipes