expose swh-graph API at archive.s.o/api/1/graph/

We would like to expose the swh-graph API to the public, by mounting it at a novel /graph endpoint of the Web API.

Caveat: as it is currently easy to DoS the swh-graph API (e.g., by requesting a full BFS visit of a significant part of the archive), we want for now to only support using that API to selected users, e.g., via their authentication tokens.

This require some discussion of which components will be responsible for what, between at least: the swh-graph API server, the swh-web API server, a reverse proxy between the two, and the authentication/authorization backend.

So, discuss :-)

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Migrated from T2589 (view on Phabricator)