Add workflow inputs and sign workflows

[LourensVeen]

Workflow inputs currently have to be assets. In many cases, some of the inputs will not be long-term data sets but ephemeral values, e.g. a script that a user wrote herself (for compute-to-data) or some configuration file for a compute asset. There needs to be a way for the user to supply these, for the sites executing the corresponding step to obtain them, and for all this to be authorised.

• Add new asset type input
• Let users upload inputs to the internal REST API
  • Separate endpoint from assets, for easier security and because we need to return a unique ID generated by the site
• Make signed sub-certificates for the user to sign workflows with
• Have some component that tracks currently active workflows, and can say whether a request for an input is valid
• Update WorkflowOrchestrator to register workflow with that component
• Check requests for inputs on the external REST API against active workflows