[BE] Multiple value streams: Switch between value streams MVC

This is the backend portion for switching between value streams. frontend work is being tracked in #221204 (closed)

Problem to Solve

This issue focuses on allowing users to switch between their saved value streams:

• Broken down from: #219106 (closed)
• Job-to-be-done:
  • Switch between saved views: When managing multiple value streams within a mono-repo, I would like to switch between my saved views, so that I can dive into the details of each individual value stream.

Intended users

• Parker (Product Manager)
• Delaney (Development Team Lead)

Solution

• MVC uses a dropdown to switch between value streams
• Display value streams that exist within the same group
• Display saved value streams in the dropdown list alphabetically
• If there aren't multiple value streams to switch between, display Create new value stream button
  • Creating a new value stream is handled in: #221202 (closed)

Dropdown to switch between value streams	Create new value stream

See Figma file

Permissions and Security

• Permissions for creating new Value Streams is the same as those for configuring them generally.

Feature Flag

• Make available to gitlab-org group on gitlab.com
• Make more broadly available after we get feedback from internal users and addressed any concerns about scale

What does success look like, and how can we measure that?

• Adoption and qualitative feedback from internal users
• Identify issues with VSA calculations, missing features, configuration options & new feature ideas

What is the type of buyer?

• This feature will be made available in GitLab Ultimate