[VSA] Add support for project-level default value stream (BE)

Problem to solve

For project-level VSA, we need to support a default value stream. However, we don't want to use the group-level endpoint because it may have non-default value streams.

Proposal

Introduce a dummy response for project-level VSA as a short term solution.

Related links

• This issue was prompted by the discussion at &5194 (comment 561537658)