Support custom inputs

[ds300]

We should let folks define async functions that can return an object matching {[key: string]: string | Buffer} that can be fed into manifests for tasks.

There should be a global option and a task-local option.

• globalCustomInputs on LazyConfig
• customInputs on TaskConfig

both with type

(args: {taskName: string, taskDir: string, }) => {[key: string]: string | Buffer} | Promise<{[key: string]: string | Buffer}>