provide send request logic

In order to support the chaining of backend modules, the pulsar connection handler needs to support sending requests (and receiving responses from) to other backend modules.

In part this logic can be ported from the frontend implementation here:

https://git.geomar.de/digital-earth/de-frontend-framework/-/blob/master/lib/messaging/PulsarConnection.ts

and/or the datahub implementation here (provided by @Chilipp):

https://git.geomar.de/dm/datahub/ak-datenanalyse/datahub-routines/-/blob/master/datahub/pulsar/connection.py