[VSA] Update the create value stream service to accept an array of stages

Problem to solve

As @ekigbo explained in !49094 (comment 462399055), the current VSA endpoints do not make it easy to create a value stream with an array of stages.

Proposal

Extend the value stream endpoint to accept the list of stages and create them in one go, the /value_streams endpoint should be able to handle an array of stages for common CRUD operations

• Sending a POST request to the /analytics/value_stream_analytics/value_streams with { name, stages: [...] } should create a new value stream, the stages should be created in the order they are sent. The newly created value stream data should be returned
• Sending a GET request to /analytics/value_stream_analytics/value_streams/:id should return the value stream with its stages
• Sending a PUT request to /analytics/value_stream_analytics/value_streams/:id

Further improvements

• We should no longer require the default in memory value stream stages
• All stages should be treated as custom stages