Graph API: add a (node type) result filters

In various use cases we want to perform a graph traversal and only return some of the nodes encountered, instead of all nodes visited. That can be done downstream, but it's a waste of transferred data, that might be significant. We want to add a "result filter", that allows to do that filtering server side.

The expressivity of it is up for discussion. For sure we want a "node type" filter that applies to visits that return nodes one by one and allows to return instead only nodes of a given (set of) types.

Result filters for traversals that return data other than individual nodes are more complicated and need further discussion. Ditto for filters that discriminate nodes based on properties other than node type (e.g., does it have a edge of a given type?).

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Migrated from T2981 (view on Phabricator)