Add feature flag reads to the client

[saluc28]

Two flags decide whether the rest of this library behaves the way its
documentation says, and reaching them meant fetching /api/v2/features and
parsing it by hand. Features returns the set keyed by flag key, FeatureEnabled
answers for one.

The reason this is worth an API and not a note in the README:
opengraph_extension_management governs more than the extensions endpoint.
GetShortestPath branches on it at pathfinding.go:156 in v9.5.1, and with the
flag off the server answers from the built-in AD and Azure kinds alone, so a
path across the edges of an installed schema comes back as 404 path not found.
A correct graph and a switched off feature look identical from the outside.

An unknown key is an error rather than false, because false is exactly what a
disabled flag answers. Nothing is cached, since a flag can be toggled while a
collector is running.