Support custom tasks

[renatoathaydes]

Decide and implement how to allow custom tasks to be added.

Tasks should be able to declare:

• inputs
• outputs
• action
• description
• arguments validator?

[renatoathaydes]

Some stuff still todo:

On JBuild side:

• look for @JbConfigProperty in parameter names when generating manifest.
• allow type JBuildLogger to be injected.
• allow complex types for properties requested with @JbConfigProperty (e.g. dependencies).

On jb:

• use @JbConfigProperty value when provided.
• handle JBuildLogger in constructor args.
• validate all keys of the config map were used.
• use task's dependents.