graph API: add endpoint to return the adjacency list of a node

The current graph API doesn't allow to simply retrieve the edges related to a given node. We could add a non recursive option to existing methods, but given how foundamental the operation is I think it deserves a dedicated method.

Concrete proposal:

• method: /graph/neighbors/:src
• return: list of node PIDs directly linked to :src via single edges
• query params:edges, direction with the usual semantics and default values

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Migrated from T1886 (view on Phabricator)