`workflowId` should be `id` in posted and returned workflow configuration

[dlongley]

When creating a workflow, the configuration for that workflow may include, optionally, the ID for that workflow under the property id. The bug currently says that property is workflowId and it should be changed to id:

https://github.com/w3c-ccg/vc-api/blob/494f0a2927a7c6766730a6b4f18c8d28daa15131/exchanges.yml#L210

Similarly, the spec currently says that workflowId will be optionally returned, when the entire config can be optionally returned, which will include the workflow ID in the id property of the configuration object (just like how it can be optionally posted using that property).