gh-35864: Return connected subgraphs with exactly k vertices
    
Fixes #35857 -- This PR adds a parameter to
`connected_subgraph_iterator` so it returns only the subgraphs of order
k.
- Added an optional parameter 'exactly_k' to connected_subgraph_iterator
- when exactly_k is True return only graphs of size k.
- As the connected subgraphs are built up one vertex at a time
(basically a DFS) we don't lose much if we are looking for subgraphs
that are small in comparison to the original graph.

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URL: #35864
Reported by: deinst
Reviewer(s): David Coudert

Author: Release Manager