[VSA] Add Value Stream GraphQL mutations

Create mutations for creating, updating and deleting value streams:

• createValueStream
• updateValueStream (by containerFullPath + id)
• deleteValueStream (by containerFullPath + id)

Input arguments for create and update:

• parentFullPath (I'm open for a better name, it will contain the full path to the project or group)
• name

Example:

updateValueStream(input: { parentFullPath: "gitlab-org", id: "ValueStream/12", name: "new name") {
}