gateway selector

closes #62 (closed)

closes #65 (closed)

This MR refactors the gateway and bridge selection. it removes the /gateway and /bridge endpoint and unifies the logic for sorting and filtering of /gateways and /bridges

The MR also changes the logic for the bucket filtering. As discussed in our in-person meeting and documented here: #62 (comment 1244458) we will return only private endpoints in case an auth token is sent and not public ones in addition to it.

The client API stays the same for /gateways and /bridges, however we need to update the clients and make sure they don't use the deprecated endpoints.