Change the state of a signature request.

Use this operation to transition a stateful signature request to a new state.

Possible transitions are:

• from DRAFT to PENDING: start the signature request: the platform wait for the activation date (if any) and transition to IN_PROGRESS
  
• from PENDING to DRAFT: suspend the signature request: allow it to be updated
  
• from PENDING to CANCELED: cancel the signature request without waiting for the activation date
  
• from IN_PROGRESS to CLOSED: close the signature request without waiting for all signatures to be colleted
  
• from IN_PROGRESS to CANCELED: cancel the signature request before all signatures get colleted
  
  Note that stateless signature requests can only be transitioned to CLOSED (TODO: explain)