Improve support for GPACT events

This PR improves support for GPACT events in the message observer. Specifically, it introduces the following changes:

adding a finalised event watcher for GPACT events (CROS-86)
supporting removed event handling for the real-time GPACT event watcher (CROS-90)
refactoring of event watcher modules, for improved code reuse and maintainability (CROS-82)
increasing test coverage (CROS-87)
update code documentation (CROS-84)

Consensys / gpact