High-level handlers MR5: Integrate with ucam_faas top-level module
This MR is split out from the original High-level handlers MR !25 (closed), towards uis/devops/epics#321
These MRs are chained, with each MR targeting the previous MR's branch. They either need to be merged in order, or all at once from the final one (after changing it to target main).
This MR integrates the high-level handlers added by previous MRs with the
ucam_faas module. Until now, the new functionality has only been available by
importing submodules. The commonly-used, public parts of the submodules are now
exported from ucam_faas by importing them there and adding to __all__.
This also integrates the new handlers with long-running mode, with support
for running the @cloud_event_handler and @message_handler handlers from
CloudEvent JSON events stored in buckets.
Issues
This MR contributes to #33, but does not remove the existing handler decorators.