Turn interface backends into an extension system

[svniemeijer]

In order to support tasktable/joborder variants that differ significantly from the MMFI/EEGS standards, it is useful to have a plugin mechanism where such tailoring can be implemented. This would be a similar to the way archive backends are handled.

The code is already to a large extend structured to allow this (since MMFI and EEGS are already considered two internal 'backends').
What needs to happen is:

• change interface backend identifier from a string to a module reference (e.g. MMFI will become 'pvml.mmfi`)
• move all MMFI/EEGS specific configuration options into a interfaceOptions sub-element in the configuration (and have these configuration options initialised by the backend instead of in config.py)

Since this will be a big backward compatibility breaking change this will mean that this will have to become a PVML V5.

[svniemeijer]

This is a bit more involved:

• we need to revisit ProductTypeConfig, with the ability to have both archive backend and interface backend options.
• we have a cross product in terms of interfaces between archive backends and interface backends that we need to resolve (e.g. the current resolve_mmfi function on the archive backend, and a similar (not yet implemented) function for the EEGS backend).
• the list of pre-defined inputs in the config might need to move to the interface backend.