"pluggable" backends, transports and formatters

[deltics]

Currently all Formatter, Backend and Transport implementations rely on non-exported implementation details. As a result, any additional Formatter, Backend and Transport implementations must be introduced in the ulog module itself.

It should be possible for new implementations to be developed external to ulog itself and added via configuration.

This will involve significant but careful refactoring of the current internal implementation details to export and expose the required extension points in a safe and reliable manner.