load the workflow config

[oliver-sanders]

Load the workflow config (as needed) in order to satisfy requests which cannot be resolved off of the data store alone.

For example:

• static graph visualization cylc-ui#82
  • The workflow config is needed to build nodes and edges for the graph view to render.
• cylc show for past and future tasks cylc-flow#5677
  • The workflow config is needed in order to match provided IDs against non-pool tasks.

We need to be very careful to ensure this doesn't cause resource issues as the workflow config can be rather large. Ideally we wouldn't load the whole workflow config (because this created runtime objects that we don't need), but unfortunately the nature of the config does not allow for this.

Only attempt this after we have an interface for tracking what workflow(s) users are current subscribed to (see #585)