Allow to create value streams in GraphQL

What does this MR do and why?

Introduce GraphQL type for value streams and a mutation to create value streams.

For now, the only supported parameter is name, on follow-up MRs we will accept parameters for stages and settings.

part of #410340 (closed)

How to set up and validate locally

1. Make sure you have premium license
2. Create a namespace(project or group) and write its full path
3. Execute the following mutation on GraphQL explorer

mutation createValueStream {
  valueStreamCreate(input: {
    namespacePath: "PATH_TO_NAMESPACE",
    name: "Test Value Stream"
  }) {
    valueStream {
      id
      name
      namespace {
        name
      }
    }
    errors
  }
}

MR acceptance checklist

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