feat: improve API + testing

[paullatzelsperger]

This PR improves on a few key aspects of the SDK:

• adds the /dataflows/:id/start endpoint
• changes API handler methods to require ids in their signatures
• adds a new message type DataFlowStartByIdMessage that contains only the sourceDataAddress
• uses chi in tests, for proper URL path handling, which the standard HTTP router can't do
• invalid state transitions now add the ErrInvalidTransition error to their tree
• terminate and suspend operations on the SDK now accept an optional reason parameter
• updated all samples